Stop //third_party/libxml from using global filter for sources

set_sources_assignment_filter() is considered as a mis-feature
of gn (as it makes it difficult to reason about the BUILD.gn
files individually).

Change code in //third_party/libxml to check the platform before
adding plaform-specific source files to "sources" variable and
clear the filter to prevent regressions.

Bug: 1018739
Change-Id: Iba4ae10ec163ce8b7ba3774cd47ebba7ad92f29a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2416350
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807987}
GitOrigin-RevId: 80c67edf96903619fcec047bbc57043445ff8a50