Add -fsigned-char for cross platform signed chars (#10852)

## 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

compute/compute-node.Dockerfile

@@ -162,7 +162,7 @@ FROM build-deps AS pg-build
 ARG PG_VERSION
 COPY vendor/postgres-${PG_VERSION:?} postgres
 RUN cd postgres && \
-    export CONFIGURE_CMD="./configure CFLAGS='-O2 -g3' --enable-debug --with-openssl --with-uuid=ossp \
+    export CONFIGURE_CMD="./configure CFLAGS='-O2 -g3 -fsigned-char' --enable-debug --with-openssl --with-uuid=ossp \
     --with-icu --with-libxml --with-libxslt --with-lz4" && \
     if [ "${PG_VERSION:?}" != "v14" ]; then \
         # zstd is available only from PG15