selftests/bpf: Fix usdt multispec failure with arm64/clang20 selftest build

When building the selftest with arm64/clang20, the following test failed:
  ...
  ubtest_multispec_usdt:PASS:usdt_100_called 0 nsec
  subtest_multispec_usdt:PASS:usdt_100_sum 0 nsec
  subtest_multispec_usdt:FAIL:usdt_300_bad_attach unexpected pointer: 0xaaaad82a2a80
  torvalds#471/2   usdt/multispec:FAIL
  torvalds#471     usdt:FAIL

But arm64/gcc11 built kernel selftests succeeded. Further debug found arm64/clang
generated code has much less argument pattern after dedup, but gcc generated
code has a lot more.

Check usdt probes with usdt.test.o on arm64 platform:

with gcc11 build binary:
  stapsdt              0x0000002e       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x00000000000054f8, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[sp]
  stapsdt              0x00000031       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x0000000000005510, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[sp, 4]
  ...
  stapsdt              0x00000032       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x0000000000005660, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[sp, 60]
  ...
  stapsdt              0x00000034       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x00000000000070e8, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[sp, 1192]
  stapsdt              0x00000034       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x0000000000007100, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[sp, 1196]
  ...
  stapsdt              0x00000032       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x0000000000009ec4, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[sp, 60]

with clang20 build binary:
  stapsdt              0x0000002e       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x00000000000009a0, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[x9]
  stapsdt              0x0000002e       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x00000000000009b8, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[x9]
  ...
  stapsdt              0x0000002e       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x0000000000002590, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[x9]
  stapsdt              0x0000002e       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x00000000000025a8, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[x8]
  ...
  stapsdt              0x0000002f       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x0000000000007fdc, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[x10]

There are total 300 locations for usdt_300. For gcc11 built binary, there are
300 spec's. But for clang20 built binary, there are 3 spec's. The default
BPF_USDT_MAX_SPEC_CNT is 256, so bpf_program__attach_usdt() will fail for gcc
but it will succeed with clang.

To fix the problem, do not do bpf_program__attach_usdt() for usdt_300
with arm64/clang setup.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250624211802.2198821-1-yonghong.song@linux.dev

Author: Yonghong Song

tools/testing/selftests/bpf/prog_tests/usdt.c

@@ -270,8 +270,16 @@ static void subtest_multispec_usdt(void)
 	 */
 	trigger_300_usdts();
 
-	/* we'll reuse usdt_100 BPF program for usdt_300 test */
 	bpf_link__destroy(skel->links.usdt_100);
+
+	bss->usdt_100_called = 0;
+	bss->usdt_100_sum = 0;
+
+	/* If built with arm64/clang, there will be much less number of specs
+	 * for usdt_300 call sites.
+	 */
+#if !defined(__aarch64__) || !defined(__clang__)
+	/* we'll reuse usdt_100 BPF program for usdt_300 test */
 	skel->links.usdt_100 = bpf_program__attach_usdt(skel->progs.usdt_100, -1, "/proc/self/exe",
 							"test", "usdt_300", NULL);
 	err = -errno;
@@ -282,13 +290,11 @@ static void subtest_multispec_usdt(void)
 	/* let's check that there are no "dangling" BPF programs attached due
 	 * to partial success of the above test:usdt_300 attachment
 	 */
-	bss->usdt_100_called = 0;
-	bss->usdt_100_sum = 0;
-
 	f300(777); /* this is 301st instance of usdt_300 */
 
 	ASSERT_EQ(bss->usdt_100_called, 0, "usdt_301_called");
 	ASSERT_EQ(bss->usdt_100_sum, 0, "usdt_301_sum");
+#endif
 
 	/* This time we have USDT with 400 inlined invocations, but arg specs
 	 * should be the same across all sites, so libbpf will only need to