mesa: remove _mesa_ffs(), implement ffs() for non-GNU platforms

mesa: remove _mesa_ffs(), implement ffs() for non-GNU platforms

Call ffs() and ffsll() everywhere.  Define our own ffs(), ffsll()
functions when the platform doesn't have them.

v2: remove #ifdef _WIN32, __IBMC__, __IBMCPP_ tests inside ffs()
implementation.  The #else clause was recursive.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Tested-by: Alexander von Gluck <kallisti5@unixzen.com>