[obj2yaml] - MIPS: move and improve testing of the e_flags

Mips/elf-flags.yaml and Mips/elf-abi.yaml are tests that intention was to
show that yaml2obj/obj2yaml are able to read/dump MIPS specific e_flags.

They were not complete, contained an excessive YAML parts and were placed
at a wrong location.

I removed them and created the obj2yaml/elf-mips-eflags.yaml instead.

Differential revision: https://reviews.llvm.org/D65807

llvm-svn: 368140