x86: unindent most of set_cpu_arch()

x86: unindent most of set_cpu_arch()

Inverting the initial if()'s condition allows to move out the bulk of
the function by a level, improving readability at least a bit. While
doing that also pull the push/pop handling up first, such that "else if"
after "return" isn't needed anymore; the order in which special cases
are checked doesn't really matter.