When generating an Assembler, you currently have two choices: Either
let the Assembler allocate a growable internal buffer, which is owned
by the Assembler. Or provide an externally allocated buffer, which
cannot grow.
This CL changes this interface to allow providing any implementation of
a buffer. The provided buffer can be a view to an externally owned
buffer, which still can grow.
This will be used to split WebAssembly compilation and code submission.
The buffer needs to be able to grow, but cannot be owned by the
Assembler because it has to survive until the code is submitted.

R=mstarzinger@chromium.org

Bug: v8:8689
Change-Id: Ib6c5ebffc8b71d0778944abac34f02c5cc7dbd79
Reviewed-on: https://chromium-review.googlesource.com/c/1411347
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58848}