arm: default to softvfp on armv6 or later cores

arm: default to softvfp on armv6 or later cores

From armv6 onwards a lot of cores started to come with a physical VFP
implementation; but many still did not and in some cases there are
both variants.  For the cores that lacked a physical VFP we would fall
back to FPU_NONE if the platform/ABI did not mandate something else.
To make matters worse, FPU_NONE is internal state used to imply
soft-fpa (ie a mixed-endian double format), so any use of .double in
hand-written assembly is almost certainly generating incorrect output.

That's undesirable, all these cores should really default to a softvfp
model.