aarch64: libgcc: Honor disable-werror [PR117600]

aarch64: libgcc: Honor disable-werror [PR117600]

In commit r15-4417-g71c7b446b98aa5, I made -werror mandatory when
building libgcc for aarch64.

While it achieved its goal (make us fix problems unnoticed so far),
there has a been a lot of debate because it couldn't be disabled
easily.

This patch adds support for --enable-werror/--disable-werror in
libgcc, defaulting to --enable-werror for aarch64.

Tested on non-bootstrap builds on aarch64-linux-gnu (with
-Wno-prio-ctor-dtor removed in order to get an error).

libgcc/ChangeLog:

	PR libgcc/117600
	* Makefile.in (WERROR): New.
	* config/aarch64/t-aarch64: Handle WERROR.
	* configure: Regenerate.
	* configure.ac: Add support for --enable-werror.