[AArch64] Add -mmark-bti-property flag.

Writing the .note.gnu.property manually is error prone and hard to
maintain in the assembly files.
The -mmark-bti-property is for the assembler to emit the section with the
GNU_PROPERTY_AARCH64_FEATURE_1_BTI. To be used when C/C++ is compiled
with -mbranch-protection=bti.

This patch refactors the .note.gnu.property handling.

Reviewed By: chill, nickdesaulniers

Differential Revision: https://reviews.llvm.org/D81930