[Mips] Support MIPS N64 relocation record format

N64 ABI relocation record r_info field in fact consists of five subfields:
* r_sym   - symbol index
* r_ssym  - special symbol
* r_type3 - third relocation type
* r_type2 - second relocation type
* r_type  - first relocation type

Up to three these relocations applied one by one. The first relocation
uses an addendum from the relocation record. Each subsequent relocation
takes as its addend the result of the previous operation. Only the final
operation actually modifies the location relocated. The first relocation
uses as a reference symbol specified by the `r_sym` field. The third
relocation assumes NULL symbol.

The patch represents these data using LLD model and takes in account
additional relocation types during a relocation calculation.

Additional relocations do not introduce any new relations between two
atoms and just specify operations need to be done during a relocation
calculation. The first relocation type (`r_type`) stored in the
`Reference::_kindValue`. The rest of relocations and `r_ssym` value are
stored in the new `Reference::_tag` field "as-is". I decided to do not
"decode" these data on the core LLD level to prevent pollution of the
core LLD model by very target specific data.

Also I have to override writing of relocation records in the `RelocationTable`
class to convert MIPS N64 ABI relocation information from the `Reference`
class back to the ELF relocation record.

http://reviews.llvm.org/D8533

llvm-svn: 233057