MIPS: Fix compilation failure on ARM in r12685.

Port r12686 (bdda5c81)

BUG=
TEST=

Review URL: https://codereview.chromium.org/11143029
Patch from Akos Palfi <palfia@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12738 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/gdb-jit.cc

@@ -1112,6 +1112,8 @@ class DebugInfoSection : public DebugSection {
       w->Write<uint8_t>(DW_OP_reg6);  // and here on x64.
 #elif defined(V8_TARGET_ARCH_ARM)
       UNIMPLEMENTED();
+#elif defined(V8_TARGET_ARCH_MIPS)
+      UNIMPLEMENTED();
 #else
 #error Unsupported target architecture.
 #endif