GN: support 32-bit x86 builds

Take over Ubuntu -x86- bots.

Note the change to Build-Win-MSVC-x86-Release-GN.json, which is not incidental.
We'll want target_cpu="x86" there too.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2340463008

Committed: https://skia.googlesource.com/skia/+/1949386ae00485ae92dd5608a2614b9dee417542

CQ_INCLUDE_TRYBOTS=master.client.skia.compile:Build-Ubuntu-Clang-x86-Debug-GN_Android-Trybot,Build-Ubuntu-GCC-x86-Release-Trybot

Review-Url: https://codereview.chromium.org/2340463008
7 files changed