The platform specific macro assembler headers can not be included
directly. They require symbols declared in macro-assembler.h.
We also cannot include macro-assembler.h from the platform specific
headers, because that would form a cycle, and the include in
macro-assembler.h would be skipped, which then also fails.

This CL documents and enforces this unfortunate situation.
This helps with further iwyu cleanups.

Note that current code which includes the platform specific headers
only works because we transitively included macro-assembler.h already
before.

R=mstarzinger@chromium.org

Bug: v8:8238, v8:7490
Change-Id: I2dc65ad950400941406e1f2f8969d0d15f524bf8
Reviewed-on: https://chromium-review.googlesource.com/c/1340240
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57578}