Make SIGRTMIN hide the real-time signals we use internally.

__SIGRTMIN will continue to tell the truth. This matches glibc's
behavior (as evidenced by the fact that we don't need a special case
in the strsignal test now).

Change-Id: I1abe1681d516577afa8cd39c837ef12467f68dd2
diff --git a/libc/bionic/posix_timers.cpp b/libc/bionic/posix_timers.cpp
index 59933ec..312bf9e 100644
--- a/libc/bionic/posix_timers.cpp
+++ b/libc/bionic/posix_timers.cpp
@@ -52,7 +52,7 @@
 // end up with one of our POSIX timer threads handling it (meaning that the intended recipient
 // doesn't). glibc uses SIGRTMIN for its POSIX timer implementation, so in the absence of any
 // reason to use anything else, we use that too.
-static const int TIMER_SIGNAL = SIGRTMIN;
+static const int TIMER_SIGNAL = (__SIGRTMIN + 0);
 
 struct PosixTimer {
   __kernel_timer_t kernel_timer_id;