amd64: 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.

Reviewed by:		kib
Differential Revision:	https://reviews.freebsd.org/D34212

sys/amd64/amd64/exec_machdep.c

@@ -95,6 +95,10 @@ __FBSDID("$FreeBSD$");
 #include <machine/specialreg.h>
 #include <machine/trap.h>
 
+_Static_assert(sizeof(mcontext_t) == 800, "mcontext_t size incorrect");
+_Static_assert(sizeof(ucontext_t) == 880, "ucontext_t size incorrect");
+_Static_assert(sizeof(siginfo_t) == 80, "siginfo_t size incorrect");
+
 /*
  * Send an interrupt to process.
  *