[RISCV] Implement support for bf16 truncate/extend on hard FP targets

For the same reasons as D151284, this requires custom lowering of the
truncate libcall on hard float ABIs (the normal libcall code path is
used on soft ABIs).

The extend operation is implemented by a shift just as in the standard
legalisation, but needs to be custom lowered because i32 isn't a legal
type on RV64.

This patch aims to make the minimal changes that result in correct
codegen for the bfloat.ll tests.

Differential Revision: https://reviews.llvm.org/D151663