Use d14 and d15 as scratch VFP registers.

Using d0 and d1 is problematic because they're argument registers
and so can end up being clobbered by instructions inserted in the
middle of a call sequence.

d14 and d15 on the other hand are OK because they don't have any
particular meaning in the ABI, and we don't even use them in the
register allocator at the moment anyway.