use runtime cache line size detection

use runtime cache line size detection

Modern glibc can easily return the L1 cache line size with
sysconf(3), so we'll use that and avoid paying a size penalty on
CPUs with smaller cache lines than 128 (every modern x86 except
the idiotic P4).

Additionally, if we detect a single CPU machine, avoid paying
any padding penalty at all.

On machines without the non-portable glibc sysconf(3)
enhancements, we'll continue to operate on the assumption
of an enormous 128 byte cache line size.