fix linking on raspbian buster

[BrettRD]

Raspbian buster still doesn't have atomics in the standard symbol table, this is a continuation of #178

discussion: #171 (comment)

This pull request only adds libatomic where it was necessary to build foxy on a pi4 using colcon build --symlink-install --packages-select rcutils

[BrettRD]

This change seems a bit too brittle and verbose, pulling foxy latest to merge it turned up a bunch of new linker errors.

Could this be converted into flag that is conditionally added to a linker list and passed to all targets?

[BrettRD]

This is the tip of the iceberg, every package that links against rcutils needs extra flags.
After patching both rcutils and rcpputils (in the same way), the build stops at 5 packages failed: class_loader fastrtps libyaml_vendor rcl_logging_spdlog rosidl_runtime_c, all missing the same linker flags, I can only assume there are more in the 122 packages colcon couldn't reach.

I don't have this issue on Armbian, just Raspi OS.
It would be nice to have clean builds on the raspberry pi, but until either it gets fixed upstream (Raspi OS) or the raspi link step goes into ROS2 testing, this patch is going to stop working for pi users very quickly.

I'm going to leave this patch here and pass flags to colcon manually.

[BrettRD]

This patch is an ineffective bandaid for a problem with the Raspbian build system.
Closing

see: #171 (comment) for workarounds (and then run a clean build)