x86: drop cpu_arch_tune_flags

x86: drop cpu_arch_tune_flags

Apparently from its introduction the variable was only ever written (the
only read is merely to determine whether to write it with another value).
(Since, due to the need to re-indent, the adjacent lines setting
cpu_arch_tune need touching anyway, switch to using PREOCESSOR_*
constants where applicable, to make more obvious what the resulting
state is going to be.)