Remove memcpy compatibility for glibc < 2.14

Summary:
This makes memcpy having undefined behavior when memory overlaps,
while memmove (previously aliased) used to be safer in this regard.
Hopefully UBSAN will catch this kind of issues.

Depends on D6342.

Test Plan:

ninja all check-all
make check

Run the Gitian builds.

Run the tests with UBSAN.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6352