RISC-V: test for expected / no unexpected symbols

RISC-V: test for expected / no unexpected symbols

Both the temporary workaround for PR/gas 29940 and the existing special
casing of GPRs in my_getSmallExpression() aren't really tested anywhere
(i.e. with the workarounds remove testing would still succeed). Nor is
there any test for uses of symbols with names matching GPRs, where such
is permitted. Before altering how this is to be dealt with, install two
testcases covering the expected behavior. (For now this includes only
known affected insns; re-ordering of entries in riscv_opcodes[] could,
however, yield more of them.)