Added notes on using 100% new files in test applications.

[Carlos O'Donell]

Added my notes on how I test applications that
might need crt file changes, or how I build
static/pie test apps using an intermediate glibc
build directory.

https://sourceware.org/glibc/wiki/Testing/Builds#Building_with_completely_new_files

The alternatives are slightly more complex.
I'm sure the script could also get upgrades to
use the right multilib based on CFLAGS.

Hopefully this helps any other more advanced
usages like wanting to compile statically to
test an app somewhere else.

-- 
Cheers,
Carlos.