Fix libcorkscrew for x86 targets (bionic) as well as the host (glibc).

Longer term, we might want to offer <ucontext.h> and have that be like glibc's,
but not today. (Note that POSIX says nothing about mcontext_t and glibc,
Mac OS, and the Linux kernel headers are all mutually incompatible.)

Change-Id: Ia074b51f4567cd54e06bbe29721389c8d1278614