Use _WIN32 instead of WIN32 in component_export.h

component_export.h was using defined(WIN32) to detect Windows for
dllimport/dllexport. However, some embedders like Skia do not define
WIN32 in their build configuration, whereas the compiler always defines
_WIN32 on Windows.

This caused component builds of partition_alloc in Skia on Windows to
fail to link (e.g. allocator_core.dll failing to link due to missing
symbols from allocator_base.dll) because symbols were not being
correctly exported.

Using _WIN32 (which is predefined by the compiler) fixes this issue
without requiring embedders to explicitly define WIN32.

Also update PRESUBMIT.py to remove WIN32 from ALLOWED_DEFINITIONS as
it is no longer used, and update build_config.md documentation.

Change-Id: I0f359790b8b57216eed6ec7098f8165c865a3142
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7902491
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Auto-Submit: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1642222}
NOKEYCHECK=True
GitOrigin-RevId: cfd45f42d4e68f36b3f06b811b2b3c4b3fabf09a