kbuild: use -MMD instead of -MD to exclude system headers from dependency

[ Upstream commit 30a7729771731971839cc969d2a321e6ea7a144b ]

This omits system headers from the generated header dependency.

System headers are not updated unless you upgrade the compiler. Nor do
they contain CONFIG options, so fixdep does not need to parse them.

Having said that, the effect of this optimization will be quite small
because the kernel code generally does not include system headers
except <stdarg.h>. Host programs include a lot of system headers,
but there are not so many in the kernel tree.

At first, keeping system headers in .*.cmd files might be useful to
detect the compiler update, but there is no guarantee that <stdarg.h>
is included from every file. So, I implemented a more reliable way in
the previous commit.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Stable-dep-of: 02e9a22ceef0 ("kbuild: hdrcheck: fix cross build with clang")
Signed-off-by: Sasha Levin <sashal@kernel.org>