Move the declarations from thread-win32.h into thread.h
and remove the macro redirection from thread-posix.h.
This will be required by following cleanups.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210614233143.1221879-4-richard.henderson@linaro.org>