[RISCV][test] Add additional RUN lines to half-convert.ll in preparation for D151824

There wasn't previous coverage for rv32id-ilp32, rv64id-lp64,
rv32id-ilp32d, or rv64id-lp64d. This is needed as D151284 fixes a bug
related to the ABI used for libcalls for fp<->fp16 conversion when hard
FP support is present.