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
diff --git a/libc/bionic/pthread.c b/libc/bionic/pthread.c
index da3a551..a6c1908 100644
--- a/libc/bionic/pthread.c
+++ b/libc/bionic/pthread.c
@@ -2106,8 +2106,8 @@
  * the C library ABI, so perform a little runtime translation here.
  */
 typedef union {
-    sigset_t   bionic;
-    uint32_t   kernel[2];
+    sigset_t           bionic;
+    __kernel_sigset_t  kernel;
 } kernel_sigset_t;
 
 /* this is a private syscall stub */
@@ -2124,8 +2124,8 @@
     kernel_sigset_t  in_set, *in_set_ptr;
     kernel_sigset_t  out_set;
 
-    in_set.kernel[0] = in_set.kernel[1] = 0;
-    out_set.kernel[0] = out_set.kernel[1] = 0;
+    memset(&in_set.kernel, 0, sizeof(in_set.kernel));
+    memset(&out_set.kernel, 0, sizeof(out_set.kernel));
 
     /* 'in_set_ptr' is the second parameter to __rt_sigprocmask. It must be NULL
      * if 'set' is NULL to ensure correct semantics (which in this case would