roll cast toolchain

  - clang 3.9 -> clang "9"  (pre-release)
  - libstdc++ -> libc++     (pretty sure)

PS1 should fail to build... need to update
infra/bots/recipe_modules/build/chromecast.py too.

PS2 compiles but fails to link.

PS3 puts -Lchromebook_arm_gles/lib on the link line;
unclear to me how we're linking today...

PS4 removed a workaround for the older toolchain.

PS5+ tries to request the older glibc math symbols
that shipped with the devices we test on... we don't
seem to have glibc 2.27 there.

Cq-Include-Trybots: skia.primary:Test-Chromecast-Clang-Chorizo-CPU-Cortex_A7-arm-Debug-All,Perf-Chromecast-Clang-Chorizo-GPU-Cortex_A7-arm-Release-All,Test-Chromecast-Clang-Chorizo-GPU-Cortex_A7-arm-Release-All
Change-Id: I7a1ac2da646fb5b0cbdeb44310f1c0747013f3f0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/234956
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
5 files changed