Define the NaCl toolchain "clang_newlib_arm" with some refactoring.
This makes it possible to build ppapi_nacl_tests on ARM, so enable that.
Also let enable_nacl_browsertests=true function for machines other than
x86-64, since that build more or less works now for all NaCl machines.
R=dpranke@chromium.org
BUG=
Review URL: https://codereview.chromium.org/1366443002
Cr-Commit-Position: refs/heads/master@{#350280}