due to 80-bit vs. 64-bit precision leading to floating point
differences when using the native code generator.  -fvia-C uses the
-ffloat-store gcc sledgehammer to avoid this.

Patch amended by Simon Marlow:
    - Test addForeignPtrFinalizerEnv too

(in the 6.8 branch only)

use extra_clean, and avoid top-level runCmd