MachO embedded: default to soft float without issuing warning

This is a simpler rule, broadly in line with previous Darwin (which chose
between "soft" and "softfp") but probably safer. In practice the only real
reason for "softfp" is ABI compatibility, not usually an issue on limited chips
like these, so anyone who wanted hard-float should already be saying so.

That's my story and I'm sticking to it.

rdar://problem/15887493