build: Fix source generation races

meson is actually executing the header and cpp generation scripts
multiple times. Since they do not co-depend on each other, they can
sometimes write the files at the same time causing subtle corruption.

Instead, just run a single generation step and split out the sources
based on header vs cpp file.

Google-Bug-Id: 505800610
Change-Id: I0d15bc958f65413df0c0f1bf73574918225d74dc
Signed-off-by: William A. Kennington III <wak@google.com>
2 files changed