Summary:
Updates the library build tests to reflect that legacy format support is now disabled by default. Also adds a new test case to verify that legacy support can still be explicitly enabled via ZSTD_LEGACY_SUPPORT=5.
Test Plan:
Run `bash tests/libzstd_builds.sh` on a Linux environment.
Tests that libzstd.so doesn't have the exec-stack bit set using
readelf. If the stack is marked executable systemd will refuse
to link against zstd. We now test that it isn't set on every PR.
Adds a test for PR #2857
Fixes Issue #2865
Yann Collet