aarch64: libgcc: Use -Werror

This patch adds -Werror to LIBGCC2_CFLAGS so that aarch64 can catch
warnings during bootstrap, while not impacting other targets.

The patch also adds -Wno-prio-ctor-dtor to avoid a warning when
compiling lse_init.c

	libgcc/
	* config/aarch64/t-aarch64: Always use -Werror
	-Wno-prio-ctor-dtor.