jit: Use std::mutex instead of pthread_mutex_t

This allows JIT to be built with a different thread model from posix
where pthread isn't available

By renaming the acquire_mutex () and release_mutex () member functions
to lock() and unlock() we make the playback::context type meet the C++
Lockable requirements. This allows it to be used with a scoped lock
(i.e. RAII) type as std::lock_guard. This automatically releases the
mutex when leaving the scope.

Co-authored-by: LIU Hao <lh_mouse@126.com>

gcc/jit/ChangeLog:

	* jit-playback.cc (playback::context::scoped_lock): Define RAII
	lock type.
	(playback::context::compile): Use scoped_lock to acquire mutex
	for the active playback context.
	(jit_mutex): Change to std::mutex.
	(playback::context::acquire_mutex): Rename to ...
	(playback::context::lock): ... this.
	(playback::context::release_mutex): Rename to ...
	(playback::context::unlock): ... this.
	* jit-playback.h (playback::context): Rename members and declare
	scoped_lock.
	* jit-recording.cc (INCLUDE_PTHREAD_H): Remove unused define.
	* libgccjit.cc (version_mutex): Change to std::mutex.
	(struct jit_version_info): Use std::lock_guard to acquire and
	release mutex.

gcc/ChangeLog:

	* system.h [INCLUDE_MUTEX]: Include header for std::mutex.