Add script to build many glibc configurations

[Zack Weinberg]

On 11/09/2016 11:27 AM, Joseph Myers wrote:
> This patch adds a Python (3.5 or later) script to build many different
> configurations of glibc, including building the required cross
> compilers first.  It's not intended to change any patch testing
> requirements, although some people may wish to use it for high-risk
> patches such as adding warning options ...

Since this does its own glibc checkout, it's not clear to me how one
should use it to test a patch(set).  I presume that whatever one does,
it only affects the "glibcs" step, but what actually do you do?  Do you
manually update /some/where/src/glibc to contain the code you want
tested and then run "glibcs", or do you somehow tell
build-many-glibcs.py the name of a branch you want tested, or what?

Thanks,
zw