libcorkscrew: avoid future name collisions.

This is a forward-compatibility patch used to avoid two problems:

1/ The C library <signal.h> is going to be updated to define 'struct sigcontext'
   properly soon. Avoid redefining this structure here when it's not really needed
   to prevent a type conflict.

2/ Similarly, proper ucontext_t support is going to be added, prevent conflict by
   using a macro renaming trick. Mainly because there is a slight chance that the
   C library definition will follow a slightly different layout/naming.

For context, see details at:

  http://code.google.com/p/android/issues/detail?id=34784
  https://android-review.googlesource.com/#/c/38875/1

Change-Id: Ie94eb5d77297f8e0bafd146b65a648bd8805a551
2 files changed