[llvm-readobj] Print MIPS .MIPS.options section content

.MIPS.options section specifies miscellaneous options to be applied
to an object file. LLVM as well as modern versions of GNU tools emit
the only type of the options - ODK_REGINFO. The patch teaches llvm-readobj
to print details of the ODK_REGINFO and skip contents of other options.

llvm-svn: 268478