[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