qemu-lock.h: Remove non-pthreads spinlock implementations

qemu-lock.h: Remove non-pthreads spinlock implementations

Since configure guarantees us that we have pthreads on all hosts
except mingw (which doesn't support a USER_ONLY config), we can
and should use the pthread_mutex based implementation of spin_lock()
and spin_unlock() in all USER_ONLY cases. This means that all the
inline-native-assembly code supporting the "USER_ONLY but not USE_NPTL"
case can go away.

The not-USER_ONLY case remains as empty implementations; there is
no change in behaviour here.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>