Only copy mcontext data from sigcontext.

The ucontext_t data structure could be bigger than the kernel data
structure. Since the unwinder only cares about the mcontext data, only
copy that out of the structure. The mcontext data is the same size in
the kernel and in the ucontext_t structure.

Bug: 15189014
Change-Id: I5978169c4425b8212e11db85a57eb319cd0e264b