Make soft-float powerpc swapcontext restore the signal mask (bug 21045).

The soft-float powerpc version of swapcontext does not restore the
signal mask, resulting in stdlib/tst-setcontext2 failing:

after getcontext
after setcontext
after swapcontext
FAIL: SIGUSR2 is blocked after swapcontext.

This patch fixes this by adjusting the arguments passed to
__sigprocmask so that it restores the saved signal mask as well as
saving the existing one.  (For hard-float, this code is only used for
a compat symbol, not for the current version of swapcontext.)

Tested for soft-float powerpc.

	[BZ #21045]
	* sysdeps/unix/sysv/linux/powerpc/powerpc32/swapcontext-common.S
	(__CONTEXT_FUNC_NAME): Pass address of signal mask to be restored
	to __sigprocmask.