GN: Harmonize ARM compiler settings with GYP

GN: Harmonize ARM compiler settings with GYP

This changes various defaults about the ARM compiler setup so
that GN matches what GYP does today:
* Prefix for cross tools is arm-linux-gnueabihf- not arm-linux-gnueabi-
* arm_float_abi defaults to hard for non-Android ARMv7
* Non-CrOS ARM builds use the ARM sysroot by default
  (the one installed by install-build-deps.sh --arm)
* Define a clang_arm toolchain, pass it the -no-integrated and
  -target arm-linux-gnueabihf and switches as GYP does

R=dpranke@chromium.org, sbc@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1352313002

Cr-Commit-Position: refs/heads/master@{#350100}