tools/build: Unbreak bmake bootstrap on Linux

Currently make.py has a hack to add the cross-build headers to the
include search path when bootstrapping bmake on Linux (but not macOS).
This is a bit of an abuse of these headers, and e9ba1fd5eda2 was not
prepared for this, since sys/bitcount.h won't exist in that instance (it
gets copied into WORLDTMP during the legacy build). Work around this
until we can wean the bmake bootstrap off using these headers by not
including sys/bitcount.h when it doesn't exist.

Fixes: e9ba1fd5eda2 ("tools/build: Provide FreeBSD's bitstring API when cross-building")