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.

Change-Id: Ie78c48690a4ce61c50593f6c39639be7fead3596
14 files changed