Fix build on Mac OSX Sierra

Fix build on Mac OSX Sierra

Summary:
There are two changes here.
The first is to eliminate `detail::DEFAULT_CLOCK_ID` from `Benchmark.[cpp|h]` as Sierra defines `clockid_t` as an enum type, which means that calling `clock_gettime(detail::DEFAULT_CLOCK_ID` would fail, because the enums are incompatible. As this was being used as a default, but is not actually changable anywhere, I just got rid of `detail::DEFAULT_CLOCK_ID` entirely.
The second is to move `portability/BitsFunctexcept.cpp` into `libfollybase_la_SOURCES`, because it's needed for generating the fingerprint tables.

Reviewed By: yfeldblum

Differential Revision: D4004843

fbshipit-source-id: b2a9c33f8e516d8eb3cdc5ab093f4946ac9ed37e