x86: improve the help text of X86_EXTENDED_PLATFORM

Change the CONFIG_X86_EXTENDED_PLATFORM help text to display the
32bit/64bit extended platform list. This is as suggested by Ingo.

Signed-off-by: Ravikiran Thirumalai <kiran@scalex86.org>
Cc: shai@scalex86.org
Cc: "Benzi Galili (Benzi@ScaleMP.com)" <benzi@scalemp.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>