PrintRegisters() should print output to `os` argument for unification,
and in case of the function would be used by other files.

Bug: v8:10821
Change-Id: Ia825c4deaf89ec454b7c293367cfa362acd4cccc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2371543Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69664}