This moves more build configuration out of libGLESv2.gni.

Bug: angleproject:3943
Change-Id: I58293f98c917e785cdf3bb1beb706f02ee8a3801
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2405800Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>