GN: add 32-bit and Release iOS builders.

GN: add 32-bit and Release iOS builders.

And roll recipes to the point where they pass target_cpu to GN,
adding x86_64 as an alias for x64.

CQ_INCLUDE_TRYBOTS=skia.primary:Build-Mac-Clang-arm64-Debug-GN_iOS,Build-Mac-Clang-arm64-Release-GN_iOS,Build-Mac-Clang-arm-Debug-GN_iOS,Build-Mac-Clang-arm-Release-GN_iOS

Change-Id: I1933d5803ec7f59f78576c5a7b16489362905a97
Reviewed-on: https://skia-review.googlesource.com/7403
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>