[CMAKE] Get rid of add_compiler_flags_to_var

Summary:
The last callsite is in the check_linker_flags function. Adding the
logic at the callsite allow to remove this function from the API.

There is no change in behavior.

Depends on D4961.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5002