Makefile: don't create a ".build/.build/" for cocci, fix output

Makefile: don't create a ".build/.build/" for cocci, fix output

Fix a couple of issues in the recently merged 0f3c55d4c2b (Merge
branch 'ab/coccicheck-incremental' into next, 2022-11-08):

In copying over the "contrib/coccinelle/" rules to
".build/contrib/coccinelle/" we inadvertently ended up with a
".build/.build/contrib/coccinelle/" as well. We'd generate the
per-file patches in the former, and keep the rule and overall result
in the latter. E.g. running:

	make contrib/coccinelle/free.cocci.patch COCCI_SOURCES="attr.c grep.c"

Would, per "tree -a .build" yield the following result:

	.build
	├── .build
	│   └── contrib
	│       └── coccinelle
	│           └── free.cocci.patch
	│               ├── attr.c
	│               ├── attr.c.log
	│               ├── grep.c
	│               └── grep.c.log
	└── contrib
	    └── coccinelle
	        ├── FOUND_H_SOURCES
	        ├── free.cocci
	        └── free.cocci.patch

Now we'll instead generate all of our files in
".build/contrib/coccinelle/". Fixing this required renaming the
directory where we keep our per-file patches, as we'd otherwise
conflict with the result.

Now the per-file patch directory is named e.g. "free.cocci.d". And the
end result will now be:

	.build
	└── contrib
	    └── coccinelle
	        ├── FOUND_H_SOURCES
	        ├── free.cocci
	        ├── free.cocci.d
	        │   ├── attr.c.patch
	        │   ├── attr.c.patch.log
	        │   ├── grep.c.patch
	        │   └── grep.c.patch.log
	        └── free.cocci.patch

The per-file patches now have a ".patch" file suffix, which fixes
another issue reported against 0f3c55d4c2b: The summary output was
confusing. Before for the "make" command above we'd emit:

	[...]
	MKDIR -p .build/contrib/coccinelle
	CP contrib/coccinelle/free.cocci .build/contrib/coccinelle/free.cocci
	GEN .build/contrib/coccinelle/FOUND_H_SOURCES
	MKDIR -p .build/.build/contrib/coccinelle/free.cocci.patch
	SPATCH .build/.build/contrib/coccinelle/free.cocci.patch/grep.c
	SPATCH .build/.build/contrib/coccinelle/free.cocci.patch/attr.c
	SPATCH CAT $^ >.build/contrib/coccinelle/free.cocci.patch
	CP .build/contrib/coccinelle/free.cocci.patch contrib/coccinelle/free.cocci.patch

But now we'll instead emit (identical output at the start omitted):

	[...]
	MKDIR -p .build/contrib/coccinelle/free.cocci.d
	SPATCH grep.c >.build/contrib/coccinelle/free.cocci.d/grep.c.patch
	SPATCH attr.c >.build/contrib/coccinelle/free.cocci.d/attr.c.patch
	SPATCH CAT .build/contrib/coccinelle/free.cocci.d/**.patch >.build/contrib/coccinelle/free.cocci.patch
	CP .build/contrib/coccinelle/free.cocci.patch contrib/coccinelle/free.cocci.patch

I.e. we have an "SPATCH" line that makes it clear that we're running
against the "{attr,grep}.c" file. The "SPATCH CAT" is then altered to
correspond to it, showing that we're concatenating the
"free.cocci.d/**.patch" files into one generated "free.cocci.patch" at
the end.

Reported-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>