clang build script: Omit unwind tables on mac/linux, build 32-bit asan runtime with clang-cl on win.

Neither should have an effect on the functionality of the compiler.

Reduces archive size:
- on Linux by 1.8MB (50.01MB -> 48.16MB)
- on Mac by 900 kB (27.71MB -> 26.32MB)
- on Windows by 1.7MB (44.78MB -> 43.06MB)

Bug: 958921
Change-Id: Ic386740fb4b07b5e72a8ab8d50fcfdc18467eaf6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1597170
Auto-Submit: Nico Weber <thakis@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657152}