Select bdver2 instead of bdver1 if TBM support is present on models < 0x10.

Tested that the right -target-cpu is set in the clang -cc1 command line
when running "clang -march=native -E -v - </dev/null" on both an FX-8150
and an FX-8350. Both are family 15h; the FX-8150 (Bulldozer processor)
reports a model number of 1, and the FX-8350 (Piledriver processor)
reports a model number of 2.