arch/tile: support signal "exception-trace" hook

This change adds support for /proc/sys/debug/exception-trace to tile.
Like x86 and sparc, by default it is set to "1", generating a one-line
printk whenever a user process crashes.  By setting it to "2", we get
a much more complete userspace diagnostic at crash time, including
a user-space backtrace, register dump, and memory dump around the
address of the crash.

Some vestiges of the Tilera-internal version of this support are
removed with this patch (the show_crashinfo variable and the
arch_coredump_signal function).  We retain a "crashinfo" boot parameter
which allows you to set the boot-time value of exception-trace.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

arch/tile/kernel/compat_signal.c

@@ -317,7 +317,7 @@ long compat_sys_rt_sigreturn(struct pt_regs *regs)
 	return 0;
 
 badframe:
-	force_sig(SIGSEGV, current);
+	signal_fault("bad sigreturn frame", regs, frame, 0);
 	return 0;
 }
 
@@ -431,6 +431,6 @@ int compat_setup_rt_frame(int sig, struct k_sigaction *ka, siginfo_t *info,
 	return 0;
 
 give_sigsegv:
-	force_sigsegv(sig, current);
+	signal_fault("bad setup frame", regs, frame, sig);
 	return -EFAULT;
 }