[RISCV] Rewrite tests to use llvm-mc/llvm-objdump

Previously the tests in lld for RISC-V were given in yaml format as
LLVM's RISC-V assembler support was incomplete. Now that the
assembler/disassembler has matured we can rewrite all tests to use
LLVM's tools.

Differential Revision: https://reviews.llvm.org/D60415

llvm-svn: 357996