gallivm: Use new LLVM fast-math-flags API

gallivm: Use new LLVM fast-math-flags API

LLVM 6 changed the API on the fast-math-flags:
https://reviews.llvm.org/rL317488

NOTE: This also enables the new flag 'ApproxFunc' to allow for
approximations for library functions (sin, cos, ...). I'm not completly
convinced, that this is something mesa should do.

Signed-off-by: Tobias Droste <tdroste@gmx.de>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-and-Tested-by: Michel Dänzer <michel.daenzer@amd.com>