enabled aggressive decommit by default

TCMALLOC_AGGRESSIVE_DECOMMIT=f is one way to disable it and
SetNumericProperty is another.
dropped atopmicops workaround for irrelevant Opteron locking bug

It's not cheap at all when done in this way (i.e. without runtime
patching) and apparently useless.

It looks like Linux kernel never got this workaround at all. See
bugzilla ticket: https://bugzilla.kernel.org/show_bug.cgi?id=11305

And I see no traces of this workaround in glibc either.

On the other hand, opensolaris folks apparently still have it (or
something similar, based on comments on linux bugzilla) in their code:
https://github.com/illumos/illumos-gate/blob/32842aabdc7c6f8f0c6140a256cf42cf5404fefb/usr/src/uts/i86pc/os/mp_startup.c#L1136

And affected CPUs (if any) are from year 2008 (that's 6 years now).

Plus even if somebody still uses those cpus (which is unlikely), they
won't have working kernel and glibc anyways.
made AtomicOps_x86CPUFeatureStruct hidden

So that access to has_sse2 is faster under -fPIC.