Coccinelle: make DEBUG_FILE option more useful

Make coccicheck checked for the existence of DEBUG_FILE on each semantic
patch, and bailed if it already existed.  This meant that DEBUG_FILE was
useless for checking more than one semantic patch at a time.  Now the check
is moved to the start of make coccicheck, and the 2> is changed to a 2>> to
append to the file on each semantic patch.  Furthermore, the spatch command
that is run for each semantic patch is also added to the DEBUG_FILE, to
make clear what each stdout trace corresponds to.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>