[CMAKE] Refactor the AddCompilerFlags facilities

Summary:
This adds a bit of code factorization by adding a pair of functions to
add and remove flags to target variable.
There is no behavior change.

Depends on D3800 and D3801.

Test Plan:

mkdir buildcmake && cd buildcmake
cmake -GNinja ..
ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3802