GN build: replace variables from magma.gni with GN paths Reduces dependencies on magma.gni Change-Id: I01648f5f23a3c45877e65ae31085ff86fb471a3a Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/1010495 Reviewed-by: John Bauman <jbauman@google.com> Commit-Queue: Craig Stout <cstout@google.com>