release: Add security/export checks to gitian and fix current failures

- fix parsing of BIND_NOW with older readelf
- add _IO_stdin_used to ignored exports

For details see: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=634261#109

- add check-symbols and check-security make targets

These are not added to the default checks because some of them depend on
release-build configs.

- always link librt for glibc back-compat builds

glibc absorbed clock_gettime in 2.17. librt (its previous location) is safe to
link in anyway for back-compat.

Fixes #7420

- add security/symbol checks to gitian

Github-Pull: #7424
Rebased-From: cd27bf51e0 475813ba5b f3d3eaf78e a8ce872118 a81c87fafc

contrib/devtools/security-check.py

@@ -94,7 +94,7 @@ def check_ELF_RELRO(executable):
         raise IOError('Error opening file')
     for line in stdout.split('\n'):
         tokens = line.split()
-        if len(tokens)>1 and tokens[1] == '(BIND_NOW)':
+        if len(tokens)>1 and tokens[1] == '(BIND_NOW)' or (len(tokens)>2 and tokens[1] == '(FLAGS)' and 'BIND_NOW' in tokens[2]):
             have_bindnow = True
     return have_gnu_relro and have_bindnow