graphics/mesa-devel: Disable "-mtls-dialect=gnu2" when built with clang 19

Clang 19 now supports "-mtls-dialect=gnu2" but it does not work for us.
Originally, I tried to use "-ftls-model=initial-exec" but it caused
regressions. Disable the optimization for now until we have a proper
implementation.

Approved by: jbeich (maintainer)
PR: 282322