Deprecate/remove riscv64sf

Is anyone using riscv64sf?  All of the existing RISC-V boards include hard-float
support as well as QEMU.  The FPGA cores we use at Cambridge also all support
hard-float.  My understanding is that glibc doesn't bother supporting soft-float
on RV64.  If no one is using it (and has no plans to use it), then I propose
we drop it in 14.0 and save one more buildworld from make tinderbox.

-- 
John Baldwin