kbuild: fix duplicated flags in DEBUG_CFLAGS

kbuild: fix duplicated flags in DEBUG_CFLAGS

Sedat Dilek noticed duplicated flags in DEBUG_CFLAGS when building
deb-pkg with CONFIG_DEBUG_INFO. For example, 'make CC=clang bindeb-pkg'
reproduces the issue.

Kbuild recurses to the top Makefile for some targets such as package
builds.

With commit 06057e9be591 ("kbuild: Only add -fno-var-tracking-assignments
for old GCC versions") applied, DEBUG_CFLAGS is now reset only when
CONFIG_CC_IS_GCC=y.

Fix it to reset DEBUG_CFLAGS all the time.

Fixes: 06057e9be591 ("kbuild: Only add -fno-var-tracking-assignments for old GCC versions")
Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: Mark Wielaard <mark@klomp.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>