i386: Clear MCE flag on AMD K6

It reports machine check capability in CPUID, but doesn't actually
implement all the necessary MSRs of the standard Intel machine
check architecture.

This fixes a boot failure on K6s recently introduced.

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