gallium/util: use #pragma section instead of #pragma data_seg

gallium/util: use #pragma section instead of #pragma data_seg

They apparently both declare the section, but #pragma data_seg
also puts all subsequent definitions in the section, which is
undesirable.

This should be the correct solution, and is actually used by the
reference I cited (but I forgot to do it in my code).

Untested, let me know if it doesn't work.