Generate genlex rules from bp2build for cc targets

This change will cause bp2build to generate genlex targets any
time a .l or .ll file is present in the srcs for a cc target and
add those genlex targets to the srcs attribute of the original
target.

Bug: 207408632
Test: unit tests
Change-Id: I1bce82c9d3c3d458eae1cef547ffae3d6e975134