Fix sanitizer stack traces on aarch64.

[rsundahl]

Summary

The bp (base pointer) variable was being loaded from register LR and
not FP on aarch64 (except for this narrow case):
defined(IPHONE_8_0) && IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_8_0

Without a valid bp from the FP register, it is not possible to traverse
previous frames for a complete stack trace. The rationale for fetching
the LR as the bp for all cases except above is not clear but since the
FP register is the canonical register for use as the frame pointer, this
commit removes the restriction above for unconditional use all aarch64.

[rsundahl]

Fixed markup issue (double underlines) in summary.

[yln]

arm64 and iOS support were introduced at the same time, but without mention why a special case is necessary:
https://reviews.llvm.org/D10510

One possible explanation for not using FP could be -fomit-frame-pointer. Is this option enabled on higher optimization levels? Do we always implicitly do -fno-omit-frame-pointer even on higher optimization levels to increase debuggability on iOS/Darwin?

Please also add a test or specify which existing test this fixes (on macOS/arm64).

[yln]

compiler-rt/lib/sanitizer_common/sanitizer_mac.cpp

910

Note to self: this is already a Darwin-specific file, so we are not changing behavior for any other platforms other than "non-iOS Darwin".

compiler-rt/lib/sanitizer_common/sanitizer_mac.cpp

 #else
   #define AARCH64_GET_REG(r) (uptr)ucontext->uc_mcontext->__ss.__##r
 #endif
 
 static void GetPcSpBp(void *context, uptr *pc, uptr *sp, uptr *bp) {
   ucontext_t *ucontext = (ucontext_t*)context;
 # if defined(__aarch64__)
   *pc = AARCH64_GET_REG(pc);
-#   if defined(__IPHONE_8_0) && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_8_0
   *bp = AARCH64_GET_REG(fp);
-#   else
-  *bp = AARCH64_GET_REG(lr);
-#   endif
   *sp = AARCH64_GET_REG(sp);
 # elif defined(__x86_64__)
   *pc = ucontext->uc_mcontext->__ss.__rip;
   *bp = ucontext->uc_mcontext->__ss.__rbp;
   *sp = ucontext->uc_mcontext->__ss.__rsp;
 # elif defined(__arm__)
   *pc = ucontext->uc_mcontext->__ss.__pc;
   *bp = ucontext->uc_mcontext->__ss.__r[7];