[CI] Enable the undefined behaviour sanitizer

Summary:
This diff enables UBSAN build.
It requires running with clang as UBSAN support in gcc is very poor (at
least until most recent versions ; in particular it won't run on Debian
9 which is our current CI target).

Test Plan: Run the CI build-ubsan config.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3749