[oe] Changing a machine def and rebuilding tool chain?

[Jeff Rush]

I'm working on a machine definition for the old TuxScreen hardware as I
haven't seen one already defined in the OE metadata.

Initially I had a:

   TARGET_CC_ARCH = "-march=armv4t -mtune=strongarm"

where the TuxScreen CPU is a StrongARM 1100.  After building a
statically-lined "helloworld" program however, it gives me an instruction
error when I try to run it on the target hardware.

Figuring it was my use of "armv4t" I changed it to just "-march=armv4
-mtune=strongarm".  Doing a:

  $ bitbake -c clean helloworld
  $ bitbake helloworld

produces the exact same binary, so obviously I need to rebuild something
further upstream in my toolchain.  Does the BitBake system notice that a
machine definition has changed, or that a TARGET_CC_ARCH has changed and
rebuild the necessary pieces?  Or do I have to blow it all away and rebuild
everything whenever something as fundamental as the TARGET_CC_ARCH changes?

Thanks,

-Jeff