kbuild: use LDFLAGS_MODULE only for .ko links

Sam Ravnborg pointed out that Documentation/kbuild/makefiles.txt already
says this is what it's for.  This patch makes the reality live up to the
documentation.  This fixes the problem of LDFLAGS_BUILD_ID getting into too
many places.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>