GCC: bring back experimental support for amdfam10/barcelona CPUs.

Initial support for the AMD amdfam10 chipsets has been available in the
gcc43 branch under GPLv2. AMD and some linux distributions (OpenSUSE) did
a backport of the amdfam10 support and made it available.

This is a revised subset of the support initially brought in in r236962
and later reverted. The collateral efects seem to have disappeared but
it is still recommended to set the CPUTYPE with caution.

Reviewed by:	jkim (ages ago)
MFC after:	3 weeks