[libre-riscv-dev] bfloat16

[lkcl]

https://libre-riscv.org/ztrans_proposal/#khronos_equiv

ah.  someone kindly informed me about the IEEE754-2019 standard being 
released a few months ago, so i updated the table to include IEEE754.

i hadn't realised that IEEE754 has such an extensive (and similar) set of 
functions.  this strengthens the case for these being scalar RV opcodes 
because, well, they're part of IEEE754.

also, whoops, i also didn't realise that rootn (and others), n is 
*integer*.  this led me to properly investigate, and there are a couple of 
operations that were overlooked: pown, powr, rootn (but interestingly no 
"root" in IEEE754-2019, despite lack of accuracy at the extreme ends).

yes those are present as opencl extended instructions:
https://www.khronos.org/registry/spir-v/specs/unified1/OpenCL.ExtendedInstructionSet.100.html