[PATCH v2] Implement run-built-tests=no for make xcheck, always build xtests

[Joseph Myers]

This shows that nptl/tst-setuid2 in fact fails to build for 
sparcv9-linux-gnu (undefined __NR_setresuid), so now breaking the build 
in build-many-glibcs-py for that configuration.

-- 
Joseph S. Myers
josmyers@redhat.com