Rename struct mcontext4 to struct freebsd4_mcontext.

This matches the recent renaming of struct freebsd4_ucontext.

Reviewed by:	brooks, imp, kib
Sponsored by:	The University of Cambridge, Google Inc.
Differential Revision:	https://reviews.freebsd.org/D33880

sys/i386/include/ucontext.h

@@ -34,7 +34,7 @@
 #define	_MACHINE_UCONTEXT_H_
 
 #if defined(_KERNEL) && defined(COMPAT_FREEBSD4)
-struct mcontext4 {
+struct freebsd4_mcontext {
 	__register_t	mc_onstack;	/* XXX - sigcontext compat. */
 	__register_t	mc_gs;		/* machine state (struct trapframe) */
 	__register_t	mc_fs;

sys/sys/ucontext.h

@@ -43,7 +43,7 @@
 #if defined(__i386__)
 struct freebsd4_ucontext {
 	sigset_t	uc_sigmask;
-	struct mcontext4 uc_mcontext;
+	struct freebsd4_mcontext uc_mcontext;
 	struct freebsd4_ucontext *uc_link;
 	stack_t		uc_stack;
 	int		__spare__[8];