Turn On -DLIBCXX_ENABLE_BENCHMARKS by default.

This patch enables the `cxx-benchmarks` target by default. Note that the target
still has to be manually invoked since it isn't included in the default 'make'
rule.

This patch also gets the benchmarks building w/ GCC. The build previously
required the '-stdlib=libc++' flag but upstream patches to Google Benchmark
now allow the library to build w/ libc++ and GCC.

These changes should make the benchmarks easier to build and test.

llvm-svn: 279999