x86/Kconfig: Merge the two CONFIG_X86_EXTENDED_PLATFORM entries

There are two menu entries for X86_EXTENDED_PLATFORM, one for X86_32 and
the other for X86_64.

These entries are nearly identical, with the only difference being the
platform list in the help message.

While this structure was intended by commit

  8425091f ("x86: improve the help text of X86_EXTENDED_PLATFORM"),

there is no need to duplicate the entire config entry. Instead, provide
a little more clarification in the help message.

  [ bp: Massage. ]

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240204100719.42574-1-masahiroy@kernel.org