uml: fix processor selection

Makes UML compile on any possible processor choice. The two problems were:

*) x86 code, when 386 is selected, checks at runtime boot_cpuflags, which we
   not have.
*) 3Dnow support for memcpy() et al. does not compile currently and fixing t
   is not trivial, so simply disable it; with this change, if one selects MK
   UML compiles (while it did not).
Merged upstream.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Adrian Bunk <bunk@stusta.de>