opcodes: add new sub-mnemonic disassembler style

opcodes: add new sub-mnemonic disassembler style

When adding libopcodes disassembler styling support for AArch64, it
feels like the results would be improved by having a new sub-mnemonic
style.  This will be used in cases like:

  add    w16, w7, w1, uxtb #2
                      ^^^^----- Here

And:

  cinc   w0, w1, ne
                 ^^----- Here

This commit just adds the new style, and prepares objdump to handle
the style.  A later commit will add AArch64 styling, and will actually
make use of the style.

As this style is currently unused, there should be no user visible
changes after this commit.