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
