Clean up the sigprocmask/pthread_sigmask implementation.

Let's have both use rt_sigprocmask, like in glibc. The 64-bit ABIs
can share the same code as the 32-bit ABIs.

Also, let's test the return side of these calls, not just the
setting.

Bug: 11069919
Change-Id: I11da99f85b5b481870943c520d05ec929b15eddb

libc/Android.mk

@@ -247,6 +247,7 @@ libc_bionic_src_files := \
     bionic/seteuid.cpp \
     bionic/setlocale.cpp \
     bionic/signalfd.cpp \
+    bionic/sigprocmask.cpp \
     bionic/sigwait.cpp \
     bionic/statvfs.cpp \
     bionic/strerror.cpp \