[Mips] Make the test more tolerant to the linker output order

The LLD output in the YAML mode depends on LLD_RUN_ROUNDTRIP_TEST
environment variable. Do not check unimportant YAML items like section-name.
[Mips] Replace stderr output by the `llvm_unreachable` call

If a regular symbol has microMIPS-bit we need to stop linking. Now the
LLD does not check the `applyRelocation` return value and continues
linking anyway. As a temporary workaround use the `llvm_unreachable`
call to stop the linker.