Merge #7424: Add security/export checks to gitian and fix current failures

a81c87f release: add security/symbol checks to gitian (Cory Fields)
a8ce872 release: always link librt for glibc back-compat builds (Cory Fields)
f3d3eaf release: add check-symbols and check-security make targets (Cory Fields)
475813b release: add _IO_stdin_used to ignored exports (Cory Fields)
cd27bf5 release: fix parsing of BIND_NOW with older readelf (Cory Fields)