[magma] Add support for libmagma-gfxstream

This change adds a 'use-magma-gfxstream' meson option to link against
libmagma from the gfxstream subproject instead of fuchsia.

Bug: b/272307395
Test: Build with -Duse-magma-gfxstream=true
Change-Id: I090eaa04b689f7c345920377a7dc56256c93a20a
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/795770
Reviewed-by: Craig Stout <cstout@google.com>
2 files changed