## Problem
In multi-character keys, the GIN index creates a CRC Hash of the first 3
bytes of the key.
The hash can have the first bit to be set or unset, needing to have a
consistent representation
of `char` across architectures for consistent results. GIN stores these
keys by their hashes
which determines the order in which the keys are obtained from the GIN
index.
By default, chars are signed in x86 and unsigned in arm, leading to
inconsistent behavior across different platform architectures. Adding
the `-fsigned-char` flag to the GCC compiler forces chars to be treated
as signed across platforms, ensuring the ordering in which the keys are
obtained consistent.
## Summary of changes
Added `-fsigned-char` to the `CFLAGS` to force GCC to use signed chars
across platforms.
Added a test to check this across platforms.
Fixes: https://github.com/neondatabase/cloud/issues/23199
