[RISCV] Custom lower FP_TO_FP16 and FP16_TO_FP to correct ABI of of libcall

As introduced in D99148, RISC-V uses the softPromoteHalf legalisation
for fp16 values without zfh, with logic ensuring that f16 values are
passed in lower bits of FPRs (see D98670) when F or D support is
present. This legalisation produces ISD::FP_TO_FP16 and ISD::FP16_TO_FP
nodes which (as described in ISDOpcodes.h) provide a "semi-softened
interface for dealing with f16 (as an i16)". i.e. the return type of the
FP_TO_FP16 is an integer rather than a float (and the arg of FP16_TO_FP
is an integer). The remainder of the description focuses primarily on
FP_TO_FP16 for ease of explanation.

FP_TO_FP16 is lowered to a libcall to `__truncsfhf2 (float)` or
`__truncdfhf2 (double)`. As of D92241, `_Float16` is used as the return
type of these libcalls if the host compiler accepts `_Float16` in a test
input (i.e. dst_t is set to `_Float16`). `_Float16` is enabled for the
RISC-V target as of D105001 and so the return value should be passed in
an FPR on hard float ABIs.

This patch fixes the ABI issue in what appears to be a minimally
invasive way - leaving the softPromoteHalf logic undisturbed, and
lowering FP_TO_FP16 to an f32-returning libcall, converting its result
to an XLen integer value.

As can be seen in the test changes, the custom lowering for FP16_TO_FP
means the libcall is no longer tail-callable.

Although this patch fixes the issue, there are two open items:
* Redundant fmv.x.w and fmv.w.x pairs are now somtimes produced during
  lowering (not a correctness issue).
* Now coverage for STRICT variants of FP16 conversion opcodes.

Differential Revision: https://reviews.llvm.org/D151284