mach_extensions is sensible on iOS, but bootstrap is not available
outside of macOS. To allow mach_extensions to be used cleanly on iOS,
the bootstrap code is moved into its own macOS-specific file.
Bug: crashpad:31
Change-Id: I7bf9d5194253b563954a1e55fbf67a16f686e8ff
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/2154529
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Mark Mentovai <mark@chromium.org>