Treat fontations ffi.rs.h as explicit output

Currently, the GN build sometimes fails if GN thought it did not have
to rebuild libfontations_ffi.a but the bazel-bin folder did not have the
header ffi.rs.h that was also required.

Now the bazel_build.py script can copy multiple files and not just into
the immediate GN folder. I also added documentation to that script to
help rationalize the functionality.

Change-Id: Iadf4475fa6ccdad80e08789e4276de045350a7f2
Bug: b/40045269
Cq-Include-Trybots: luci.skia.skia.primary:Build-Debian10-Clang-x86_64-Debug-Fontations,Build-Mac-Clang-x86_64-Debug-Fontations
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/738779
Reviewed-by: Dominik Röttsches <drott@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
2 files changed