link clang bots against our prepackaged libc++

This will let ASAN bots find __cxa_thread_atexit(),
letting us clean up thread_local values at thread
exit rather than having to leak them.

Change-Id: Ifd09a1efe659744bb26adf7280bb0f95722a2325
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/266529
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
18 files changed