blockfreq: Temporarily turn on -debug-only=block-freq

These tests fail after my BlockFrequencyInfo rewrite on two buildbots
[1][2].  I can't reproduce it locally, so I'm temporarily turning on
-debug-only=block-freq so I can find the problem.

[1]: http://bb.pgr.jp/builders/ninja-x64-msvc-RA-centos6/builds/1860
[2]: http://llvm-amd64.freebsd.your.org/b/builders/clang-i386-freebsd/builds/18477