[AArch64][compiler-rt] Add Pointer Authentication support for VFORK.

The LR is stored to off-stack spill area where it is vulnerable.
"paciasp" add an auth code to the LR while the "autiasp" verifies that so
LR can't be modiifed on the spill area.

Test: build with -DCMAKE_C_FLAGS="-mbranch-protection=standard",
run on Armv8.3 capable hardware with PAuth.

Reviewed By: eugenis

Differential Revision: https://reviews.llvm.org/D98009

Author: DanielKristofKiss

compiler-rt/lib/sanitizer_common/sanitizer_common_interceptors_vfork_aarch64.inc.S

@@ -1,6 +1,7 @@
 #if defined(__aarch64__) && defined(__linux__)
 
 #include "sanitizer_common/sanitizer_asm.h"
+#include "builtins/assembly.h"
 
 ASM_HIDDEN(COMMON_INTERCEPTOR_SPILL_AREA)
 
@@ -9,6 +10,7 @@ ASM_HIDDEN(COMMON_INTERCEPTOR_SPILL_AREA)
 ASM_TYPE_FUNCTION(ASM_WRAPPER_NAME(vfork))
 ASM_WRAPPER_NAME(vfork):
         // Save x30 in the off-stack spill area.
+        paciasp
         stp     xzr, x30, [sp, #-16]!
         bl      COMMON_INTERCEPTOR_SPILL_AREA
         ldp     xzr, x30, [sp], 16
@@ -33,11 +35,14 @@ ASM_WRAPPER_NAME(vfork):
         bl     COMMON_INTERCEPTOR_SPILL_AREA
         ldr    x30, [x0]
         ldp    x0, xzr, [sp], 16
+        autiasp
 
         ret
 ASM_SIZE(vfork)
 
 .weak vfork
 .set vfork, ASM_WRAPPER_NAME(vfork)
 
+GNU_PROPERTY_BTI_PAC
+
 #endif