RISC-V: fix TARGET_PROMOTE_FUNCTION_MODE hook for libcalls

RISC-V: fix TARGET_PROMOTE_FUNCTION_MODE hook for libcalls

Fixes: 3496ca4e6566 ("RISC-V: Add runtime invariant support")
riscv_promote_function_mode doesn't promote a SI to DI for libcalls
case. It intends to do that however the code is broken (regression).

The fix is what generic promote_mode () in explow.cc does. I really
don't understand why the old code didn't work, but stepping thru the
debugger shows old code didn't and fixed does.

This showed up when testing Ajit's REE ABI extension series which probes
the ABI (using a NULL tree type) and ends up hitting the libcall code path.

gcc/ChangeLog:
	* config/riscv/riscv.cc (riscv_promote_function_mode): Fix mode
	returned for libcall case.

Tested-by: Patrick O'Neill <patrick@rivosinc.com> # pre-commit-CI #526
Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>