Fix m68k so it builds with current trunk

[Jeff Law]

It's not currently possible to build various crosses with the current 
trunk due to warning issues.  I suspect this is the first of many 
patches to address those issues.

I'm not familiar with the code in question, but based on Maxim's comment 
the gcc_unreachable ought to be safe.  It could possibly even replace 
the entire else clause here, but I choose to avoid that simply because 
it's riskier.

I've confirmed the various m68k-* targets from config-all.mk will build 
after this patch.

Installed on the trunk.

Jeff
diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 6548386..6bbb32c 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,7 @@
+2016-10-25  Jeff Law  <l...@redhat.com>
+
+       * config/m68k/m68k.c (m68k_get_reloc_decoration): Add gcc_unreachable.
+
 2016-10-25  Martin Liska  <mli...@suse.cz>
 
        PR sanitizer/78106
diff --git a/gcc/config/m68k/m68k.c b/gcc/config/m68k/m68k.c
index b152ca8..ce56692 100644
--- a/gcc/config/m68k/m68k.c
+++ b/gcc/config/m68k/m68k.c
@@ -4550,6 +4550,7 @@ m68k_get_reloc_decoration (enum m68k_reloc reloc)
                }
            }
        }
+      gcc_unreachable ();
 
     case RELOC_TLSGD:
       return "@TLSGD";