[ada, build] Avoid cp -p failures during Ada make install

[Arnaud Charlet]

> Tested on x86_64-pc-linux-gnu installing both to a local filesystem and
> an NFSv3 filesystem.
> 
> Ok for mainline (and the gcc-9 and gcc-8 branches eventually)?

No, this is not OK.

I'd rather keep the simple current logic and either stick to cp -p, or
use a proper $(INSTALL_whatever) as done elsewhere rather than adding more
kludges.

Also, standard.ads.h is a valid file, so the reference shouldn't be removed.

I'll add it to the repository, this was an oversight, thanks for noticing.

Arno