libc: Provide ucontext_t/mcontext_t/<sys/ucontext.h>

This patch updates the C library headers to provide ucontext_t
definitions for three architectures.

+ Fix <signal.h> to always define 'struct sigcontext'.

The new declarations are announced with new macros defined in
<sys/cdefs.h> in order to make it easier to adapt client code
that already defines its own, incompatible, versions of the
structures seen here.

http://code.google.com/p/android/issues/detail?id=34784

Change-Id: Ie78c48690a4ce61c50593f6c39639be7fead3596

libc/include/time.h

@@ -30,10 +30,7 @@
 
 #include <sys/cdefs.h>
 #include <sys/time.h>
-
-#define __ARCH_SI_UID_T __kernel_uid32_t
-#include <asm/siginfo.h>
-#undef __ARCH_SI_UID_T
+#include <machine/signal.h>  /* for struct sigevent */
 
 __BEGIN_DECLS