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.
d6d7afcdbc