This moves the platform-specific distinction to one place, and allows
to just include assembler-inl.h instead of macro-assembler.h to get the
correct inline functions defined in assembler.h (e.g.
RelocInfo::set_target_address).

R=mstarzinger@chromium.org, titzer@chromium.org

Review-Url: https://codereview.chromium.org/2587113002
Cr-Commit-Position: refs/heads/master@{#41843}