Allows Clang to use LLVM's fixes-x18 option

This patch allows clang to have llvm reserve the x18
platform register on AArch64. FreeBSD will use this in the kernel for
per-cpu data but has no need to reserve this register in userland so
will need this flag to reserve it.

This uses llvm r226664 to allow this register to be reserved.

Patch by Andrew Turner.