okl4: declare read-write-lock as volatile

GCC 4.7.4 and newer seems to optimize the lock-variable accesses more
radically, which uncovered the missing volatile qualifier and resulted
in:

Assertion "(int)locked >= 0" failed in file '.../okl4_x86/kernel/include/kernel/read_write_lock.h', line 151 (fn=f0104771)
--- "KD# assert" ---