add -Z soft-float option

This change adds -Z soft-float option for generating
software floating point library calls.
It also implies using soft float ABI, that is the same as llc.

It is useful for targets that have no FPU.
auto merge of #9617 : crabtw/rust/softfp, r=alexcrichton

This change adds --soft-float option for generating
software floating point library calls.
It also implies using soft float ABI, that is the same as llc.

It is useful for targets that have no FPU.