Correct definition of 'struct sigcontext' on MIPS.

Add missing fields ('sr' and 'mc_tls') to 'struct sigcontext'.

The kernel doesn't use 'struct sigcontext' but instead uses 'ucontext_t'
which includes 'mcontext_t' in 'struct sigframe' to build the signal frame.
As a result, this change is not an ABI change but simply making
'struct sigcontext' correct.  Note that 'struct sigcontext' is only used
for "Traditional BSD style" signal handlers.

While here, rename the 'xxx' field to '__spare__' to match 'mcontext_t'.

Sponsored by:	DARPA, AFRL

sys/mips/include/signal.h

@@ -68,11 +68,13 @@ struct	sigcontext {
 	int		sc_onstack;	/* sigstack state to restore */
 	__register_t	sc_pc;		/* pc at time of signal */
 	__register_t	sc_regs[32];	/* processor regs 0 to 31 */
+	__register_t	sr;		/* status register */
 	__register_t	mullo, mulhi;	/* mullo and mulhi registers... */
 	int		sc_fpused;	/* fp has been used */
 	f_register_t	sc_fpregs[33];	/* fp regs 0 to 31 and csr */
 	__register_t	sc_fpc_eir;	/* fp exception instruction reg */
-	int		xxx[8];		/* XXX reserved */ 
+	void		*sc_tls;	/* pointer to TLS area */
+	int		__spare__[8];	/* XXX reserved */ 
 };
 
 #endif /* !_ANSI_SOURCE && !_POSIX_SOURCE */