Driver: alter the getARMFloatABI signature

This changes getARMFloatABI to use the ToolChain and Args instead of Driver,
Args, Triple.  Although this pushes the Triple calculation/parsing into the
function itself, it enables the use of the function for a future change.  The
reason to sink the triple calculation here is to avoid threading the Triple
through multiple layers in a future change.

llvm-svn: 248095