Add Armv8.1-M PACBTI library variants

[pratlucas]

This adds the folowing library variants with support for the PACBTI-M architecture extension:

• armv8.1m.main_soft_nofp_nomve_pacret_bti[_exn_rtti]
• armv8.1m.main_hard_fp_nomve_pacret_bti[_exn_rtti]
• armv8.1m.main_hard_fpdp_nomve_pacret_bti[_exn_rtti]
• armv8.1m.main_hard_nofp_mve_pacret_bti[_exn_rtti]

Unfortunately, qemu currently doesn't provide support for PACBTI-M, which means these variants can't be fully tested at runtime level. Since the PACBTI-M instructions are NOP-copatible, we can use an arvm8.1m.main CPU as a workaround to ensure that the library code still works correctly regardless of branch protecion.