librsvg: set the needed atomic configuration flags directly per-target

Do not try to mangle the upstream list of targets;
after discussion with upstream it turns out it's neither
necessary nor upstreamable:
https://github.com/crossbeam-rs/crossbeam/pull/751



Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>