lib: libutils: Use ret for longjmp with Clang and BTI

longjmp uses br instead of ret to jump to the target. Thus the target
location should have the right BTI launchpad to handle this.
clang has a bug [1] and doesn't add the BTI after setjmp causing
exception when BTI is enabled. This works well with gcc [2]
and can be tested when compiling xtests with WITH_CXX_TESTS=n.
To avoid the exception, use ret instead of br with clang and BTI.

[1] - https://bugs.llvm.org/show_bug.cgi?id=49544
[2] - https://gcc.gnu.org/legacy-ml/gcc-patches/2018-11/msg02472.html

Signed-off-by: Ruchika Gupta <ruchika.gupta@linaro.org>
Reviewed-by: Jerome Forissier <jerome@forissier.org>

lib/libutils/isoc/arch/arm/setjmp_a64.S

@@ -90,8 +90,17 @@ BTI(	bti	c)
 	mov	sp, x16
 	cmp	w1, #0
 	cinc	w0, w1, eq
+/*
+ * clang has a bug and doesn't insert bti after setjmp
+ * causing BTI ecxception. Remove this when the bug is fixed.
+ * https://bugs.llvm.org/show_bug.cgi?id=49544
+ */
+#if defined(__clang__) && defined(CFG_TA_BTI)
+	ret
+#else
 	// use br not ret, as ret is guaranteed to mispredict
 	br	x30
+#endif
 	.size	longjmp, .-longjmp
 
 BTI(emit_aarch64_feature_1_and     GNU_PROPERTY_AARCH64_FEATURE_1_BTI)