Changes

Support FP reg -> int reg moves

Using instruction fmv.x.d.

This is necessary to implement the ELF psABI calling conventions,
whereas some FP arguments may have to be passed in integer registers.
 (commit: 16794b9)

Update C calling conventions to the RISC-V ELF psABI

The original implementation of loc_external_arguments and
loc_external_results was following an older ABI,
where an FP argument passed in an FP register "burns" an integer register.

In the ELF psABI, integer registers and FP registers are used independently,
as in the OCaml calling convention.  Plus, if all FP registers are used
but an integer register remains, the integer register is used to pass
the next FP argument.

Fixes: #9515
 (commit: ea6896f)