Actions: replace `$LD` with `$CC` or `$CXX`?

[woodruffw]

I had this idea while thinking about a resolution for GaloisInc/MATE#58: the C/C++ compiler frontend should have most (all?) of the same functionality as the linker it wraps, and replacing $LD invocations in a build with $CC or $CXX invocations would make nested instrumentation (via GLLVM) more accurate.