RISCV: simplify a test case for RISCV (NFCI)

The output of the object file is unimportant and entirely discarded.
Simply redirect the output to `/dev/null` or `NUL` as the case may be.
Additionally, the space between the labels is unimportant.  There is no
need to add space between the labels.  Two labels at the same address
are sufficient to generate the difference expression and should still
test the same behaviour.