toolchain/wrapper: make the {str, len} tuple more generic, add comments

toolchain/wrapper: make the {str, len} tuple more generic, add comments

In 61cb120 (toolchain/wrapper: extend paranoid check to -isystem), we
introduced a {str,len} tuple to check the various arguments passed to
gcc, to avoid hard-coding an ever-growing, long list of those args
directly in the condition check.

It was made specific to the arguments (the structure member is named
'arg'), but can also be used to store the unsafe paths as well.

Also, that piece is almost un-documented.

Rename the structure member so that it is more generic, and add a bit of
comments to explain the whole of it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>