mesa: Move compile of common Mesa core files to a static lib.

mesa: Move compile of common Mesa core files to a static lib.

We were compiling them twice, costing extra build time.  Reduces my
ccache-hot clean build time by a second (24.3s to 23.3s, 3 runs each).

The windows args are a little strange -- it's not clear to me that
they're actually used for building these files, but keep them in place
just in case, since we don't have a good windows CI story yet.  We
should want them on both gallium and classic regardless: Only osmesa
could be built for windows in classic, and classic OSMesa's scons
build defines these flags too.

Closes: #2052
Acked-by: Dylan Baker <dylan@pnwbakers.com>