Add aarch64-w64-mingw32 target to libgcc

Reuse MinGW definitions from i386 for libgcc. Move reused files to
libgcc/config/mingw folder.

libgcc/ChangeLog:

	* config.host: Add aarch64-w64-mingw32 target. Adjust targets
	after moving MinGW files.
	* config/i386/t-gthr-win32: Move to...
	* config/mingw/t-gthr-win32: ...here.
	* config/i386/t-mingw-pthread: Move to...
	* config/mingw/t-mingw-pthread: ...here.
	* config/aarch64/t-no-eh: New file. EH is not yet implemented for
	the target, and the default definition should be disabled.