[CMAKE] Add a check_linker_flag function

Summary:
Separate the check of the flag from its addition. This has the
beneficial side-effect to discard the need for removing the Werror
flag from CMAKE_REQUIRED_FLAGS, since its scope is now reduced.

There is no change in behavior.

Test Plan:

ninja check

Run the Gitian builds

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4961