x86: fix processing of -M disassembler option

x86: fix processing of -M disassembler option

Multiple -M options can be specified in any order. Therefore stright
assignment to fields affected needs to be avoided, such that earlier
options' effects won't be discarded. This was in particular a problem
for -Msuffix followed by certain of the other sub-options.

While updating documentation, take the liberty and also drop the
redundant mentioning of being able to comma-separate multiple options.