[PATCH] i386: Add support for compilation for Core2

gcc doesn't support -mtune=core2 yet, but will be soon. Use -mtune=generic or -mtune=i686
as fallback

TBD need benchmarking for INTEL_USERCOPY etc. So far I used the same defaults as MPENTIUMM

Signed-off-by: Andi Kleen <ak@suse.de>