[AArch64] Enable return address signing for static ctors

Functions generated by clang and included in the .init_array section (such as
static constructors) do not follow the usual code path for adding
target-specific function attributes, so we have to add the return address
signing attribute here too, as is currently done for the sanitisers.

Differential revision: https://reviews.llvm.org/D51418

llvm-svn: 342126