atf: Fix ATF_BUILD_* values when not using the bootstrap compiler

Currently, we encode the full path and compile flags for the build
compiler in libatf. However, these values are not correct when
cross-compiling: For example, when I build on macOS, CC is set to the
host path /usr/local/Cellar/llvm/11.0.0_1/bin/clang-11. This path will
not exist on the target system.
Simplify this logic and use cc/cpp/c++ since those binaries will exist
on the target system unless the compiler was explicitly disabled.
I'm not convinced ATF needs to encode these values, but this is a
minimal fix for these tests when using a non-bootstrapped compiler.

Reviewed By: ngie, brooks
Differential Revision: https://reviews.freebsd.org/D28414