Package flag enabling coq's native-compiler flag

This package acts as a package flag for the ambient switch, taken into account by coq (and possibly any coq library) to enable native_compute at configure time, triggering the installation of .coq-native/* files for the coq standard library and subsequent coq libraries.

This implements item 1 of CEP #48 https://github.com/coq/ceps/pull/48.

Remarks:

1. you might face with issues installing this package flag under macOS, see https://github.com/coq/coq/issues/11178.
2. this package is not intended to be used as a dependency of other packages (notably as installing or uninstalling this package may trigger a rebuild of all coq packages in the ambient switch).
3. the option set by this package will be automatically propagated to coqc for coq >= 8.13 (but the packaging of coq libraries for earlier versions of coq may need an update: for details, see item 3 of CEP #48 https://github.com/coq/ceps/pull/48).