Fix the definition of cas() on x86 (#8219)

*p is both read and written to by the cmpxchg instruction, and therefore
should be given the '+' constraint modifier.

(In GCC's extended ASM language, '+' means that the operand is both read
and written to whereas '=' means that it is only written to.)

Otherwise, the compiler is allowed to rewrite something like

SpinLock lock;
initSpinLock(&lock);       /* sets lock = 1 */
ACQUIRE_SPIN_LOCK(&lock);

into

SpinLock lock;
ACQUIRE_SPIN_LOCK(&lock);

because according to the asm statement, the previous value of 'lock' is
not important.