aarch64: Run SUBTARGET_INIT_BUILTINS if it exists

aarch64: Run SUBTARGET_INIT_BUILTINS if it exists

Some subtargets don't provide the canonical function names as
the symbol name in C libraries, and libcalls will only work if
the builtins are patched to emit the correct library name.

For example, on NetBSD, cabsl has the symbol name __c99_cabsl,
and the patching is done via netbsd_patch_builtin.

With this change, libgfortran.so is correctly built with a
reference to __c99_cabsl, instead of "cabsl" which is not defined.

gcc/ChangeLog:
	* config/aarch64/aarch64.c (aarch64_init_builtins):
	Call SUBTARGET_INIT_BUILTINS.