Detect if char equals int8_t

Summary:
This happen to be the case for SmarOS/OpenSolaris.
This is untested on these platforms as they are not officialy supported.

Backport of core PR13580
https://github.com/bitcoin/bitcoin/pull/13580/files

Test Plan:

make check
ninja check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3815