Add a new 'pc' option to -Z branch-protection for aarch64 that
enables the use of PC as a diversifier in PAC branch protection code.
When the pauth-lr target feature is enabled in combination
with -Z branch-protection=pac-ret,pc, the new 9.5-a instructions
(pacibsppc, retaasppc, etc) will be generated.
With the exception of `tests/run-make/translation/test.rs`, which has a
syntax error.
The expected output in `rustdoc-error-lines/rmake.rs`'s required slight
tweaking.
The two `reproducible-build.rs` files need `// ignore-tidy-linelength`
because rustfmt produces lines longer than 100 chars, which tidy doesn't
like, yuk.
