[ARM][libcxxabi] Add PACBTI-M support to libcxxabi

[stuij]

Summary

This change consists of just adding 'BTI' to the prologue of Arm assembly
functions, which is just the one: __cxa_end_cleanup

This patch is part of a series that adds support for the PACBTI-M extension of
the Armv8.1-M architecture, as detailed here:

https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/armv8-1-m-pointer-authentication-and-branch-target-identification-extension

The PACBTI-M specification can be found in the Armv8-M Architecture Reference
Manual:

https://developer.arm.com/documentation/ddi0553/latest

The following people contributed to this patch:

• Mikhail Maltsev

[danielkiss]

Cloud you please rebase this patch? there were a few changes to __cxa_end_cleanup recently.

[stuij]

rebased as per review request

[danielkiss]

Thanks for the rebase, LGTM

libcxxabi/src/cxa_exception.cpp

__cxa_end_cleanup_impl()
     }
     return &exception_header->unwindHeader;
 }
 
 asm("  .pushsection  .text.__cxa_end_cleanup,\"ax\",%progbits\n"
     "  .globl  __cxa_end_cleanup\n"
     "  .type __cxa_end_cleanup,%function\n"
     "__cxa_end_cleanup:\n"
+#if defined(__ARM_FEATURE_BTI_DEFAULT)
+    "  bti\n"
+#endif
     "  push  {r1, r2, r3, lr}\n"
     "  bl  __cxa_end_cleanup_impl\n"
     "  pop {r1, r2, r3, r4}\n"
     "  mov lr, r4\n"
 #if defined(LIBCXXABI_BAREMETAL)
     "  ldr r4, =_Unwind_Resume\n"
     "  bx  r4\n"
 #else