gcc: Set native_system_header_dir on aarch64-mingw

Provide a sensible default value for native_system_header_dir, namely
/mingw/include, on aarch64-mingw. This is in line with the expectations
for mingw file locations, and is already set on both x86- and
x86_64-mingw.

gcc/ChangeLog:
	* config.gcc (aarch64-*-mingw*): Set native_system_header_dir.