[AArch64] Emit CFI instruction for updating x18 when using ShadowCallStack with exception unwinding

PR45875 notes an instance where exception handling crashes on aarch64-fuchsia
where SCS is enabled by default. The underlying issue seems to be that within libunwind,
various _Unwind_* functions, the x18 register is not updated if a function is marked
with nounwind. This removes the check for nounwind and emits the CFI instruction that updates x18.

Differential Revision: https://reviews.llvm.org/D79822