Rename the (mksnapshot-specific) --ebt-os flag to --target-os. This
flag specifies the target os for mksnapshot-generated files,
specifically for embedded.S.

In the future we should base all of embedded-file-writer.cc on this
(and an upcoming --target-arch) flag and remove other current
cross-compilation workarounds (e.g.: V8_TARGET_OS_WIN,
V8_TARGET_OS_FUCHSIA).

Bug: chromium:945659,v8:9103
Change-Id: I814fa8c5d4ee9ca0542f63dcae790086d887bbe2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1559748
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60711}