[libc-eglibc] Rewrite part of the code

Rewrite part of the code to better match the rest.
Most notably, rewrite handling of:
if [ ... ] && [ ... ]
to:
if [ ... -a ... ]

This has the positive side effect of calling "[" only once, although
"[" is probably a shell built-in.

 source config/global.in

 source config/target.in

 source config/toolchain.in

 source config/kernel.in

 source config/companion_libs.in

 source config/binutils.in

 source config/cc.in

 source config/libc.in

 source config/tools.in

 source config/debug.in