[RISCV][NFC] Rework test/MC/RISCV/rv{32,64}* to allow testing of symbol operands

Standardise on check lines:
* CHECK-ASM
* CHECK-OBJ
* CHECK-ASM-AND-OBJ

This allows for the addition of tests involving symbol operands, which will
not result in identical instructions in both assembly and disassembled object 
output.

This commit doesn't exploit this reworking to increase test coverage of symbol
operands - that will come in a future patch.

llvm-svn: 341546