We extracted the platform-specific API header into separate, platform-
specific header files to avoid OS-specific includes in include/v8.h.
As the platform-specific headers are only needed on their specific
platform, we did not make them available on other platforms in BUILD.gn.
Unfortunately, this is not possible, because the `include` validation in
`gn gen --check` checks the availability of header files without
considering `#ifdef` in the source files. Therefore the check fails on
Windows when the POSIX-specific header file is not available.

With this CL platform-specific headers will be generally available. This
is no problem because they can only be included on their specific
platform anyways.

R=adamk@chromium.org

Bug: chromium:921971
Change-Id: Iee7cc22063ce31a536f937d73799b7217ac1a94e
Reviewed-on: https://chromium-review.googlesource.com/c/1443057Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59196}