Switch the arm64 pcpu to a global register variable

[andrew]

Summary

This removes an unneeded instruction to move the pointer from x18 to a
temporary register.

[emaste]

Looks fine to me

[emaste]

GCC ref: https://gcc.gnu.org/onlinedocs/gcc/Global-Register-Variables.html#Global-Register-Variables
Some recentish LLVM discussion on general named global register variables: https://lists.llvm.org/pipermail/llvm-dev/2018-December/128706.html
Related patch: https://reviews.llvm.org/D68862