clearing env for make check

[Roland McGrath]

I had a `make check' run fouled up because I had some random LD_PRELOAD
setting I didn't want in my environment (and didn't know I had).  I got rid
of that setting, since I didn't want it.  But such a setting might be
something someone actually wants for their normal program runs, and it
would be nice not to have `make check' or other things that run using the
newly-built libc not broken by it.  How about adding an option to ld.so
when invoked directly to ignore all the LD_* environment variables?  Then,
I'd change all the tests et al to pass --ignore-environment along with
--library-path.

If this sounds good, then I can implement it.