[benchmark] Stop building benchmarks by default

Although the benchmark regex-related build issue seems to be
fixed, it appears that benchmark library triggers some stage 2 clang-cl
bugs:

http://lab.llvm.org:8011/builders/clang-x86-windows-msvc2015/builds/13495/steps/build%20stage%202/logs/stdio

The only sensible option now is to prevent benchmark library from
building in the default configuration.

llvm-svn: 340836