[AArch64][compiler-rt] Strip PAC from the link register.

-mbranch-protection protects the LR on the stack with PAC.
When the frames are walked the LR need to be cleared.
This inline assembly later will be replaced with a new builtin.

Test: build with  -DCMAKE_C_FLAGS="-mbranch-protection=standard".

Reviewed By: kubamracek

Differential Revision: https://reviews.llvm.org/D98008