x86: parse special opcode modifiers for .insn

x86: parse special opcode modifiers for .insn

So called "short form" encoding is specified by a trailing "+r", whereas
a possible extension opcode is specified by the usual "/<digit>". Take
these off the expression before handing it to get_absolute_expression().

Note that on targets where / starts a comment, --divide needs passing to
gas in order to make use of the extension opcode functionality.