Builds for an iOS target were left unhandled in the v8_libbase
component inside BUILD.gn. This adds the appropriate source files to
the build for iOS targets.

Tbr: machenbach@chromium.org
Bug: v8:9140
Change-Id: I853e9a39727d80f0f281c8ac2da4eeb465e188e6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1569430Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60871}