[PATCH] x86-64: Use -mtune=generic for generic kernels

The upcomming gcc 4.2 got a new option -mtune=generic to tune
code for both common AMD and Intel CPUs. Use this option
when available for generic kernels.

On x86-64 it is used with CONFIG_GENERIC_CPU. On i386 it is
enabled with CONFIG_X86_GENERIC.  It won't affect the base
line CPU support in any ways and also not the minimum supported CPU.

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>