builtins: expand out the AEABI function stubs

These actually may change calling conventions.  We cannot simply provide
function aliases as the aliased function may have a different calling
convention.  Provide a forwarding function instead to permit the
compiler to synthesize the calling convention adjustment thunk.

Remove the `ARM_EABI_FNALIAS` macro as that is not safe to use.

Resolves PR33030!

llvm-svn: 303188