aarch64: Fix async_fibre_swapcontext() on clang BTI builds

Reverting to using swapcontext() when compiling with clang on BTI-enabled
builds fixes the BTI setjmp() failure seen when running asynctest.

The issue with setjmp/longjmp is a known clang bug: see
llvm/llvm-project#48888

Change-Id: I6eeaaa2e15f402789f1b3e742038f84bef846e29

Reviewed-by: Matt Caswell <[email protected]>
Reviewed-by: Paul Dale <[email protected]>
(Merged from #17698)

(cherry picked from commit d2d2401)

Author: tom-cosgrove-arm

crypto/async/arch/async_posix.h

@@ -36,6 +36,18 @@
  */
 #   define USE_SWAPCONTEXT
 #  endif
+#  if defined(__aarch64__) && defined(__clang__) \
+    && defined(__ARM_FEATURE_BTI_DEFAULT) && __ARM_FEATURE_BTI_DEFAULT == 1
+/*
+ * setjmp/longjmp don't currently work with BTI on all libc implementations
+ * when compiled by clang. This is because clang doesn't put a BTI after the
+ * call to setjmp where it returns the second time. This then fails on libc
+ * implementations - notably glibc - which use an indirect jump to there.
+ * So use the swapcontext implementation, which does work.
+ * See https://github.com/llvm/llvm-project/issues/48888.
+ */
+#   define USE_SWAPCONTEXT
+#  endif
 #  include <ucontext.h>
 #  ifndef USE_SWAPCONTEXT
 #   include <setjmp.h>