[AARCH64][ELF][llvm-readobj] Support for AArch64 .note.gnu.property

    
ELF for the 64-bit Arm Architecture defines a processor specific property
type GNU_PROPERTY_AARCH64_FEATURE_1_AND as GNU_PROPERTY_LOPROC. This
property works in a similar way to the existing X86 processor specific
property GNU_PROPERTY_GNU_X86_FEATURE_1_AND.

Two feature bits are defined for GNU_PROPERTY_AARCH64_FEATURE_1_AND:
- GNU_PROPERTY_AARCH64_FEATURE_1_BTI 0x1
- GNU_PROPERTY_AARCH64_FEATURE_1_PAC 0x2

This patch defines the property, feature bits and implements support for
printing in llvm-readobj.

Differential Revision: https://reviews.llvm.org/D62595

llvm-svn: 362490