arm: adjust FPU selection logic

arm: adjust FPU selection logic

The logic here seems to be overly complex, so simplify it a bit.  One
particular problem was that using the legacy -mno-fpu option was not
working properly, as this has all the feature bits set to zero causing
the code to then pick a different FPU as the default.  Fix this by
only selecting an FPU as a fallback if the code has not otherwise
selected one: there was only one route by which this could happen.

This patch is really a pre-cursor to the following one where we want
to make no-fpu internally a fall-back position for some legacy
processors where previously we would have dropped back to the FPA.