Fix unused parameter warning for mips builds

libbacktrace_offline was inheriting -Wno-unused-parameter from llvm's
makefiles.  Fix an unused parameter warning when compiling for an
unsupported architecture, and return false since *value wasn't
written.

Test: lunch aosp_mips-eng && mma -j
Change-Id: I7df990f01e1025ca17ec3717107df56d2349f3cb

libbacktrace/BacktraceOffline.cpp

@@ -34,6 +34,7 @@ extern "C" {
 #include <vector>
 
 #include <android-base/file.h>
+#include <android-base/macros.h>
 #include <backtrace/Backtrace.h>
 #include <backtrace/BacktraceMap.h>
 #include <ziparchive/zip_archive.h>
@@ -534,6 +535,10 @@ bool BacktraceOffline::ReadReg(size_t reg, uint64_t* value) {
     default:
       result = false;
   }
+#else
+  UNUSED(reg);
+  UNUSED(value);
+  result = false;
 #endif
   return result;
 }