kbuild: improve cc-option to clean up all temporary files

When cc-option and friends evaluate compiler flags, the temporary file
$$TMP is created as an output object, and automatically cleaned up.
The actual file path of $$TMP is .<pid>.tmp, here <pid> is the process
ID of $(shell ...) invoked from cc-option. (Please note $$$$ is the
escape sequence of $$).

Such garbage files are cleaned up in most cases, but some compiler flags
create additional output files.

For example, -gsplit-dwarf creates a .dwo file.

When CONFIG_DEBUG_INFO_SPLIT=y, you will see a bunch of .<pid>.dwo files
left in the top of build directories. You may not notice them unless you
do 'ls -a', but the garbage files will increase every time you run 'make'.

This commit changes the temporary object path to .tmp_<pid>/tmp, and
removes .tmp_<pid> directory when exiting. Separate build artifacts such
as *.dwo will be cleaned up all together because their file paths are
usually determined based on the base name of the object.

Another example is -ftest-coverage, which outputs the coverage data into
<base-name-of-object>.gcno
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/Kbuild.include

@@ -86,20 +86,21 @@ cc-cross-prefix = $(firstword $(foreach c, $(1), \
 			$(if $(shell command -v -- $(c)gcc 2>/dev/null), $(c))))
 
 # output directory for tests below
-TMPOUT := $(if $(KBUILD_EXTMOD),$(firstword $(KBUILD_EXTMOD))/)
+TMPOUT = $(if $(KBUILD_EXTMOD),$(firstword $(KBUILD_EXTMOD))/).tmp_$$$$
 
 # try-run
 # Usage: option = $(call try-run, $(CC)...-o "$$TMP",option-ok,otherwise)
 # Exit code chooses option. "$$TMP" serves as a temporary file and is
 # automatically cleaned up.
 try-run = $(shell set -e;		\
-	TMP="$(TMPOUT).$$$$.tmp";	\
-	TMPO="$(TMPOUT).$$$$.o";	\
+	TMP=$(TMPOUT)/tmp;		\
+	TMPO=$(TMPOUT)/tmp.o;		\
+	mkdir -p $(TMPOUT);		\
+	trap "rm -rf $(TMPOUT)" EXIT;	\
 	if ($(1)) >/dev/null 2>&1;	\
 	then echo "$(2)";		\
 	else echo "$(3)";		\
-	fi;				\
-	rm -f "$$TMP" "$$TMPO")
+	fi)
 
 # as-option
 # Usage: cflags-y += $(call as-option,-Wa$(comma)-isa=foo,)