selftests: vDSO: align stack for O2-optimized memcpy

When switching on -O2, gcc generates SSE2 instructions that assume a
16-byte aligned stack, which the standalone test's start point wasn't
aligning. Fix this with the usual alignment sequence.

Fixes: ecb8bd7 ("selftests: vDSO: build tests with O2 optimization")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202409241558.98e13f6f-oliver.sang@intel.com
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/vDSO/vdso_standalone_test_x86.c

@@ -131,6 +131,8 @@ asm (
 	"_start:\n\t"
 #ifdef __x86_64__
 	"mov %rsp,%rdi\n\t"
+	"and $-16,%rsp\n\t"
+	"sub $8,%rsp\n\t"
 	"jmp c_main"
 #else
 	"push %esp\n\t"