[PAC][llvm-readobj][AArch64][ELF] Support `GNU_PROPERTY_AARCH64_FEATURE_PAUTH` (#85231)

This adds support for `GNU_PROPERTY_AARCH64_FEATURE_PAUTH` feature (as
defined in https://github.com/ARM-software/abi-aa/pull/240) handling in
llvm-readobj and llvm-readelf. The following constants for supported
platforms are also introduced:

- `AARCH64_PAUTH_PLATFORM_INVALID = 0x0`
- `AARCH64_PAUTH_PLATFORM_BAREMETAL = 0x1`
- `AARCH64_PAUTH_PLATFORM_LLVM_LINUX = 0x10000002`

For the llvm_linux platform, output of the tools contains descriptions
of PAuth features which are enabled/disabled depending on the version
value. Version value bits correspond to the following `LangOptions`
defined in #85232:

- bit 0: `PointerAuthIntrinsics`;
- bit 1: `PointerAuthCalls`;
- bit 2: `PointerAuthReturns`;
- bit 3: `PointerAuthAuthTraps`;
- bit 4: `PointerAuthVTPtrAddressDiscrimination`;
- bit 5: `PointerAuthVTPtrTypeDiscrimination`;
- bit 6: `PointerAuthInitFini`.

Support for `.note.AARCH64-PAUTH-ABI-tag` is dropped since it's deleted
from the spec in ARM-software/abi-aa#250.