MIPS: Add tests for orphan REL HI16 relocation addend processing

Add GAS and LD test cases for orphan REL HI16 relocation processing,
including verification of the addends produced as well as the warning
messages expected.

Due to the ordering used for the list of oustanding cached HI16 reloc
entries warnings for non-ELF links are issued in the reverse order
compared to one for ELF links.  This is not considered a defect as all
the required warnings are produced in both cases, however separate
warning lists are required and included here for ELF and srec output.