x86: generate names for /proc/cpuinfo from <asm/cpufeature.h>

We have had a number of cases where <asm/cpufeature.h> (and its
predecessors) have diverged substantially from the names list in
/proc/cpuinfo.  This patch generates the latter from the former.

It retains the option for explicitly overriding the strings, but by
making that require a separate action it should at least be less
likely to happen.

It would be good to do a future pass and rename strings that are
gratuituously different in the kernel (/proc/cpuinfo is a userspace
interface and must remain constant.)

Signed-off-by: H. Peter Anvin <hpa@zytor.com>