[ELFYAML] Support mips64 relocation record format in yaml2obj/obj2yaml

MIPS64 ELF file has a very specific relocation record format. Each
record might specify up to three relocation operations. So the `r_info`
field in fact consists of three relocation type sub-fields and optional
code of "special" symbols.

http://techpubs.sgi.com/library/manuals/4000/007-4658-001/pdf/007-4658-001.pdf
page 40

The patch implements support of the MIPS64 relocation record format in
yaml2obj/obj2yaml tools by introducing new optional Relocation fields:
Type2, Type3, and SpecSym. These fields are recognized only if the
object/YAML file relates to the MIPS64 target.

Differential Revision: http://reviews.llvm.org/D7136

llvm-svn: 227044