[RISCV][test][llvm-objdump] Add test cases for objdump's handling of RISCV arch attributes

In preparation for a follow-up patch to adjust the policy. The error for
an unrecognized version of the base ISA is particularly problematic, as
binaries produced from a current GCC are rejected.

The testing approach is modeled on the riscv-attributes.s file in
lld/test/ELF.