Support win32 platform matching

[Buhlakay]

Currently toolchain_helpers.bzl assumes win_amd64 for all Windows toolchains. When using things like the i686-pc-windows-msvc 32-bit Windows toolchains, this results in 64-bit Windows wheels being incorrectly bundled.

[Buhlakay]

For context, we have some custom 32-bit Windows Python toolchains we use with cross-platform builds, so we are currently patching this - I'm happy to take a swing at this issue.

[jvolkman]

I didn't realize 32-bit windows was still a thing. Is it an easy change? Can you identify 32 vs 64-bit from that py_platform string?

[Buhlakay]

Haha yeah, the tooling is a bit dated.. My patch:

     elif py_platform.endswith("windows-msvc") or py_platform.endswith("windows-msvc-freethreaded"):
-        return ["win_amd64"]
+        if py_platform.find("i686") == -1:
+            return ["win_amd64"]
+        return ["win32"]

It seems we're already making assumptions on the naming based on what's published from https://github.com/astral-sh/python-build-standalone/releases, so that seems to follow suit.

[jvolkman]

@Buhlakay I think I would restructure a bit to assume 64-bit by default:

     elif py_platform.endswith("windows-msvc") or py_platform.endswith("windows-msvc-freethreaded"):
+        if "i686" in py_platform:
+            return ["win32"]
         return ["win_amd64"]

Can you submit a PR?