Clean up sys_signame and sys_siglist a little.

We don't need quite so much duplication because we already have a way
to get the signal number from its name, and that already copes with the
fact that the mips/mips64 numbers are different from everyone else's.

Also remove sys_signame from LP64. glibc doesn't have this BSD-ism.

Change-Id: I6dc411a3d73589383c85d3b07d9d648311492a10

libc/include/signal.h

@@ -61,7 +61,9 @@ typedef int sig_atomic_t;
 #define NSIG _NSIG
 
 extern const char* const sys_siglist[];
+#if !defined(__LP64__)
 extern const char* const sys_signame[];
+#endif
 
 typedef __sighandler_t sig_t; /* BSD compatibility. */
 typedef __sighandler_t sighandler_t; /* glibc compatibility. */