This change configures the Linux clang -Og target as a toolchain with
args. To support this, all Pigweed variables from pw_vars_default are
made into build args, defined within their respective modules.
The Linux clang target/toolchain is the only one that currently works.
Pigweed's other targets will be ported in later changes.
Change-Id: I051e29fde7577e41a3184c37031a4e04936404e1