x86: permit "default" with .arch

So far there was no way to reset the architecture to that assembly would
start with in the absence of any overrides (command line or directives).
Note that for Intel MCU "default" is merely an alias of "iamcu".

While there also zap a stray @item from the doc section, as noticed
when inspecting the generated output (which still has some quirks, but
those aren't easy to address without re-flowing almost the entire
section).