[CMAKE] Use plural form when multiple arguments are expected

Summary:
Some add_xxx_flag() functions can actually take multiple flags are
argument. Rename them to add_xxx_flags() (note the 's') makes it
clearer and more consistent with other function names.

There is no behavior change.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3801