MIPS/BFD: Fix RELA handling of borrow in the generic linker

MIPS/BFD: Fix RELA handling of borrow in the generic linker

Fix an issue with `_bfd_mips_elf_generic_reloc' not taking into account
any borrow from the lower part in the handling of relocations of the
HI/LO kind and resulting in incorrect calculations made for RELA targets
in the generic used for non-ELF output such as S-records.  This doesn't
trigger for REL targets because they call `_bfd_mips_elf_generic_reloc'
indirectly from `_bfd_mips_elf_lo16_reloc' so as to obtain a complete
32-bit addend from relocation pairs and in calculating the addend the
latter function uses a hack to work around the lack of borrow handling
in the former function.

The MIPS/ELF linker is unaffected as it uses its own calculations.

Correct the calculation of the relevant partial relocations made in
`_bfd_mips_elf_generic_reloc' then to take the borrow into account and
remove the hack from `_bfd_mips_elf_lo16_reloc' as no longer needed.

Add generic linker test cases accordingly expecting the same disassembly
from srec output produced as from ELF output produced by the MIPS/ELF
linker.