powerpc: Add static asssert for context size

Add a static assert for the siginfo_t, mcontext_t and ucontext_t
sizes. These are de-facto ABI options and cannot change size ever. For
powerpc64, also add asserts for {u,m}mcontext32_t and siginfo32.

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D34213