move u_math to src/util

move u_math to src/util

Currently we have two sets of functions for bit counts, one in gallium
and one in core mesa. The ones in core mesa are header only in many
cases, since they reduce to "#define _mesa_bitcount popcount", but they
provide a fallback implementation. This is important because 32bit msvc
doesn't have popcountll, just popcount; so when nir (for example)
includes the core mesa header it doesn't (and shouldn't) link with core
mesa. To fix this we'll promote the version out of gallium util, then
replace the core mesa uses with the util version, since nir (and other
non-core mesa users) can and do link with mesautils.

Acked-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>