Fix BUILD.gn for Fuchsia platform build. (#2692)

In order for the Fuchsia source tree to update its
version of SPIRV-Tools to a newer upstream, the
BUILD.gn needs to be slightly altered to take care
of the fact that it can be used with a different
GN //build set of rules and configs than the
Chromium one.

This is done by using |build_with_chromium|, already
defined by //build_overrides/build.gni, to guard
Chromium-specific statements.

+ Add a target to generate spirv-opt which is used by
  Fuchsia to optimize shaders at build time for some
  of its graphics libraries.