This is in preparation for better cross-compile support in mksnapshot.
Specifically, this CL series will introduce runtime switches to select
the target platform for generated embedded.S assembly.

Each platform writer will derive from the abstract base class
PlatformEmbeddedFileWriterBase. Currently, the code remains
functionally unmodified and was just moved to
PlatformEmbeddedFileWriterGeneric. This will be split up in future
CLs.

Bug: v8:9103
Change-Id: Ie7e29bb60ba5a8ff6c0c1edec676943b80a1781b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1622854
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61745}