configure: Use MSYSTEM_CARCH for default arch on msys2

On msys2, uname -m returns the architecture of the base msys2 layer. On Windows on arm64, the base msys2 layer itself is still x86_64 only, even if running with e.g. the clangarm64 where the windows native applications are built as aarch64.

If MSYSTEM_CARCH is set, use this instead of uname -m for the default architecture. This gives the correct behaviour for the clangarm64 environments. It also gives the correct default for the 32 bit x86 environments such as mingw32.