gdb: only return signal values for powerpc's gdb_cpu_signal()

Summary:
The values returned by this function are reported to the gdb client as
the reason for the break in execution, a signal value such as SIGTRAP,
SIGEMT, or SIGSEGV. As such, exact vector numbers can be misidentified.
Return SIGEMT in the default case instead.

Reviewed by:	alfredo
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D28046

sys/powerpc/powerpc/gdb_machdep.c

@@ -98,10 +98,7 @@ gdb_cpu_signal(int vector, int dummy __unused)
 		return (SIGTRAP);
 #endif
 
-	if (vector <= 255)
-		return (vector);
-	else
-		return (SIGEMT);
+	return (SIGEMT);
 }
 
 void