[AArch64][PtrAuth] Fix unwind state for tail calls

When generating unwind tables for code which uses return-address
signing, we need to toggle the RA_SIGN_STATE DWARF register around any
tail-calls, because these require the return address to be authenticated
before the call, and could throw an exception. This is done using the
.cfi_negate_ra_state directive before the call, and .cfi_restore_state
at the start of the next basic block.

However, since D153098, the .cfi_restore_state isn't being inserted,
because the CFIFixup pass isn't being run. This re-enables that pass
when return-adress signing is enabled.

Reviewed By: ikudrin, MaskRay

Differential Revision: https://reviews.llvm.org/D156428