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

[Florian Weimer]

* 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.

Sorry, I missed that due to the other persistent SPARC failure.
I'll see what I can do about it.

Thanks,
Florian