[AArch64] Emit PAC/BTI .note.gnu.property flags

This patch make LLVM emit the processor specific program property types
defined in AArch64 ELF spec
https://developer.arm.com/docs/ihi0056/f/elf-for-the-arm-64-bit-architecture-aarch64-abi-2019q2-documentation

A file containing no functions gets both property flags.  Otherwise, a property
is set iff all the functions in the file have the corresponding attribute.

Patch by Daniel Kiss and Momchil Velikov.

Differential Revision: https://reviews.llvm.org/D71019