Don't fall back to host $(CC) if the target gcc is not checked out

Error "cc1plus: error: unrecognized command line option
‘-mfloat-abi=softfp’" is much more confusing than
"...gcc: No such file or directory".
Also we don't build in Windows/Cygwin any more so we don't need the old
trick.

Change-Id: Ibbe3da67b412575e56bd6e61ef412ea5b2789fea