[Mips] Check ELF flags to prevent linking of incompatible files

1. The path checks ELF header flags to prevent linking of incompatible files.
   For example we do not allow to link files with different ABI, -mnan
   flags, some combination of target CPU etc.
2. The patch merge ELF header flags from input object files to put their
   combination to the generated file. For example, if some input files
   have EF_MIPS_NOREORDER flag we need to put this flag to the output
   file header.

I use the `parseFile()` (not `canParse()`) method because in case of
recognition of incorrect input flags combination we should show detailed
error message and stop the linking process and should not try to use
another `Reader`.

llvm-svn: 221439