Refactor NaCl IRT toolchains, use arm-nacl-clang

This factors the irt_* toolchain definitions using a template to reduce
copied boilerplate.  It also switches the ARM build to use arm-nacl-clang
rather than arm-nacl-gcc, which matches what the GYP build does now.  Many
of the compilation flags are still not at all right, but this is enough
for the build of the IRT to complete successfully.

BUG=531702
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/1341143002

Cr-Commit-Position: refs/heads/master@{#348781}

• BUILD.gn[diff]
• build/config/compiler/BUILD.gn[diff]
• build/toolchain/nacl/BUILD.gn[diff]