[AArch64] Support selecting TPIDR_EL[1-3] as the thread base

Add an -mtp=el[0-3] option to select which of the AArch64 thread ID registers
will be used for the TLS base pointer.

This is a followup to rL356657 which added subtarget features to enable
accesses to the privileged thread ID registers.

Patch by Philip Derrin!

Differential revision: https://reviews.llvm.org/D59631

llvm-svn: 357250