ANGLE has a ANGLE_IS_32/64_BIT_CPU define that is based on the gn arg
for "target_cpu".  Don't emit this define as is from the gn desc, move
it to an architecture-specific section of the generated blueprint.

BUG=angleproject:2344

Change-Id: Ib6387d5f31b30900b5d3195da6d9b21c48e1e2a4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2090393Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>