v8: fix openbsd build

This is [1] applied ahead of time. Summary:

    OpenBSD doesn't have <ucontext.h>. ucontext_t lives in <signal.h>
    and is a typedef for struct sigcontext. There is no uc_mcontext.

[1] https://codereview.chromium.org/21705003/

Note: the patch has been accepted upstream but hasn't made its way into
a stable release yet.