Port 1952f928

Original Commit Message:

    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=clemensh@chromium.org, joransiu@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: Iff6217ccb961d009a31f6adc50a7ef77ca1c8b70
Reviewed-on: https://chromium-review.googlesource.com/c/1342977Reviewed-by: Muntasir Mallick <mmallick@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#57618}