[RISCV][NFC] Expand test/MC/RISCV/linker-relaxation.s tests

Add more complete CHECK lines for the relocations generated when relaxation is
enabled, and add cases where a locally defined symbol is referenced.

Two instances of pcrel_lo(defined_symbol) are commented out, as they will
produce an error. A follow-up patch will fix this.

llvm-svn: 356734