Windows: add CONFIG_TLS_THREAD=y and swap pthreadGC2 to libwinpthread-1.

Since GCC 4.4, MinGW has supported TLS via the __thread keyword, so
generate config-host.mak with CONFIG_TLS_THREAD=y.

MinGW now uses a new threading library libwinpthread-1.dll: install it
instead of pthreadGC2.dll.

Signed-off-by: Jens Axboe <axboe@kernel.dk>