x86/cpu: Drop configuration options for early 64-bit CPUs

The x86 CPU selection menu is confusing for a number of reasons:

When configuring 32-bit kernels, it shows a small number of early 64-bit
microarchitectures (K8, Core 2) but not the regular generic 64-bit target
that is the normal default.  There is no longer a reason to run 32-bit
kernels on production 64-bit systems, so only actual 32-bit CPUs need
to be shown here.

When configuring 64-bit kernels, the options also pointless as there is
no way to pick any CPU from the past 15 years, leaving GENERIC_CPU as
the only sensible choice.

Address both of the above by removing the obsolete options and making
all 64-bit kernels run on both Intel and AMD CPUs from any generation.
Testing generic 32-bit kernels on 64-bit hardware remains possible,
just not building a 32-bit kernel that requires a 64-bit CPU.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250226213714.4040853-5-arnd@kernel.org