Clang build script: refactor some -DCOMPILER_RT_BUILD flags

Addressing comments on https://chromium-review.googlesource.com/c/chromium/src/+/1708085/14#message-a761ae3d9a7449f4c294457a2d569f0f6a24191f

Bug: 929645
Change-Id: I813506f333f823fd9df28af5282ae77c765844e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1712883
Commit-Queue: Hans Wennborg <hans@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680144}
2 files changed