[Clang][AArch64] Add ACLE macros for FEAT_PAuth_LR (#80163)

This updates clang's target defines to include the ACLE changes covering
the FEAT_PAuth_LR architecture extension.
The changes include:
* The new `__ARM_FEATURE_PAUTH_LR` feature macro, which is set to 1 when
  FEAT_PAuth_LR is available in the target.
* A new bit field for the existing `__ARM_FEATURE_PAC_DEFAULT` macro,
  indicating the use of PC as a diversifier for Pointer Authentication
  (from -mbranch-protection=pac-ret+pc).

The approved changes to the ACLE spec can be found here:
https://github.com/ARM-software/acle/pull/292