Fix bug in flx. Add C/C++ dependency generator.

Flx failed because the name of the driver (flx_run/flx_arun)
wasn't calculated when running dynamic programs. Fixed.

Add new entry points in the toolchain plugins to help calculate
dependencies of C/C++ files. Uses -M etc on gcc/clang.
We need this to reduce the rebuild time of the Felix RTL,
which currently takes O(hours) on my box because
there's no dependency checking.