Allow specifying copts for different tools invoked by xctoolrunner

[segiddins]

Xcode allows specifying some options (for e.g. momc or mapc or ibtool or actool) via various build settings (e.g. we rely on setting --no-inverse-relationship-warnings to keep builds quiet via MOMC_NO_INVERSE_RELATIONSHIP_WARNINGS in Xcode). Currently, the resource rules do no have a way to specify copts that should be given to the resource compiling tools, but it would be really nice if there was.

[keith]

Another useful case for this for actool would be to pass specific options if you want to produce thinned builds like --filter-for-device-model and --filter-for-device-os-version

[segiddins]

cc @thomasvl, in case you have any thought on how best to implement this