Auto merge of #131900 - mrkajetanp:target-feature-pauth-lr, r=Amanieu

rustc_target: Add pauth-lr aarch64 target feature

Add the pauth-lr target feature, corresponding to aarch64 FEAT_PAuth_LR. This feature has been added in LLVM 19.
It is currently not supported by the Linux hwcap and so we cannot add runtime feature detection for it at this time.

r? `@Amanieu`