cmd/internal/obj/riscv: prevent constant loads that do not target registers

Check that the target of a constant load is a register and add test coverage
for this error condition. While here, rename the RISC-V testdata and tests
to be consistent with other platforms.

Change-Id: I7fd0bfcee8cf9df0597d72e65cd74a2d0bfd349a
Reviewed-on: https://go-review.googlesource.com/c/go/+/292895


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>