[clang][ARM] re-use arm::isHardTPSupported for hardware TLS check

This conditional check for -mstack-protector-guard=tls got out of sync
with the conditional check for -mtp=cp15 by me in D114116, because I
forgot about the similar check added in D113026.

Re-use the code in arm::isHardTPSupported so that these aren't out of
sync.

Interestingly, our CI reported this when testing
-mstack-protector-guard=tls; it was only reproducible with Debian's LLVM
and not upstream LLVM due to this out of tree patch:
https://salsa.debian.org/pkg-llvm-team/llvm-toolchain/-/blob/snapshot/debian/patches/930008-arm.diff

Fixes: https://github.com/ClangBuiltLinux/linux/issues/1502

Reviewed By: ardb

Differential Revision: https://reviews.llvm.org/D116233