[ARM] reject -mtp=cp15 if target subarch does not support it

Currently, we permit -mtp=cp15 even for targets that don't implement the
TLS register. When building for ARMv6 or earlier, this means we emit
instructions that will UNDEF at runtime. For Thumb1, passing -mtp=cp15
will trigger an assert in the backend.

So let's add some diagnostics to ensure that -mtp=cp15 is only accepted
for ARMv6T2 or newer.

Reviewed By: nickdesaulniers

Differential Revision: https://reviews.llvm.org/D113026