Fix gcc version checks for _mesa_bitcount

Fix gcc version checks for  _mesa_bitcount

- Fix _GNUC__ typo in both checks
- Fix logic error in check for gcc < 3.4 that breaks for gcc 2.x & older

Without this fix, builds with gcc 3.4.x end up depending on undefined
_mesa_bitcount instead of gcc's __builtin_popcount.

NOTE: This is a candidate for the stable branches.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>