[safestack] Fast access to the unsafe stack pointer on AArch64/Android.

[safestack] Fast access to the unsafe stack pointer on AArch64/Android.

Android libc provides a fixed TLS slot for the unsafe stack pointer,
and this change implements direct access to that slot on AArch64 via
__builtin_thread_pointer() + offset.

This change also moves more code into TargetLowering and its
target-specific subclasses to get rid of target-specific codegen
in SafeStackPass.

This change does not touch the ARM backend because ARM lowers
builting_thread_pointer as aeabi_read_tp, which is not available
on Android.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@250456 91177308-0d34-0410-b5e6-96231b3b80d8