Use target_cpu instead of cpu_arch in BUILD.gn
Use target_cpu instead of cpu_arch in BUILD.gn

Also add missing defines for arm simulator build

BUG=v8:3841
R=machenbach@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/874393007

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#27164}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh