ARM: Use non-VFP softcalls on embedded Darwinish targets

The compiler-rt functions __adddf3vfp and so on exist purely to allow Thumb1
code to make use of VFP instructions by switching back to ARM mode, they make
no sense for M-class processors which don't even have an ARM mode.

Given that justification, in practice this is a platform ABI decision so the
actual check is based on that rather than CPU features.

rdar://problem/15302004