Make clang the default compiler on host.

Motivation, GCC's compiler warnings are inferior to clang's. -Wthread-safety is
not supported by GCC starting with version 4.7. As this change only effects the
host, performance issues are an impact on host building and testing alone.
Fix clang gtest building on host with BUILD_HOST_64bit.
Fix clang build regressions caused by unused fields.
Fix x86-64 regression caused by requirement to fire-up quick compiler even in an
interpret-only environment. Long-term this code doesn't belong in the quick
compiler.

Change-Id: Ifc2b10177f40d0724cbbf8dab9653ac03cdd1cee
diff --git a/compiler/dex/verification_results.cc b/compiler/dex/verification_results.cc
index 6b0875c..a7f67e7 100644
--- a/compiler/dex/verification_results.cc
+++ b/compiler/dex/verification_results.cc
@@ -30,11 +30,11 @@
 namespace art {
 
 VerificationResults::VerificationResults(const CompilerOptions* compiler_options)
-    : compiler_options_(compiler_options),
-      verified_methods_lock_("compiler verified methods lock"),
+    : verified_methods_lock_("compiler verified methods lock"),
       verified_methods_(),
       rejected_classes_lock_("compiler rejected classes lock"),
       rejected_classes_() {
+  UNUSED(compiler_options);
 }
 
 VerificationResults::~VerificationResults() {