rust_uprev: emerge arm-none-eabi-gcc & do it in parallel

arm-none-eabi-gcc isn't mentioned in the list of RUSTC_TARGET_TRIPLES,
though it's necessary (we check for it in the ebuild)

While I'm in the area, if we're unconditionally emerge'ing all of these
anyway, do it in parallel. Doing so makes installation take 1/4 as long.

BUG=chromium:1136579, chromium:1112551
TEST=Ran the script

Change-Id: I49063487b2e2b2c83e2ef30dd8f8904b13c59f38
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2462927
Reviewed-by: Tiancong Wang <tcwang@google.com>
Tested-by: George Burgess <gbiv@chromium.org>
2 files changed