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