Introduce aarch64-w64-mingw32 target

Add the initial aarch64-w64-mingw32 target for gcc.

This is the first commit in a sequence of patch series to add
new aarch64-w64-mingw32 target.

Coauthors: Zac Walker <zacwalker@microsoft.com>,
Mark Harmstone <mark@harmstone.com>  and
Ron Riddle <ron.riddle@microsoft.com>

Refactored, prepared, and validated by
Radek Barton <radek.barton@microsoft.com> and
Evgeny Karpov <evgeny.karpov@microsoft.com>

fixincludes/ChangeLog:

	* mkfixinc.sh: Extend for *-mingw32* targets.

gcc/ChangeLog:

	* config.gcc: Add aarch64-w64-mingw32 target.