thread_sync.c (rb_mutex_lock): fix deadlock

	* thread_sync.c (rb_mutex_lock): fix deadlock
	  [ruby-core:87467] [Bug #14841]

	thread_sync.c (rb_mutex_lock): acquire lock before being killed

	We (the thread acquiring the mutex) need to acquire the mutex
	before being killed to work with ConditionVariable#wait.

	Thus we reinstate the acquire-immediately-after-sleeping logic
	from pre-r63711 while still retaining the
	acquire-after-checking-for-interrupts logic from r63711.

	This regression was introduced in
	commit 501069b8 (r63711)
	("thread_sync.c (rb_mutex_lock): fix deadlock") for
	[Bug #14841]

	[ruby-core:88503] [Bug #14999] [Bug #14841]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@64998 b2dd03c8-39d4-4d8f-98ff-823fe69b080e