kern.mk: Fix wrong variable being used for linker path after 172a624f0

When I synchronized kern.mk with bsd.sys.mk, I accidentally changed
CCLDFLAGS to LDFLAGS which is not used by the kernel builds. This commit
should unbreak the GitHub actions cross-build CI. I didn't notice it
locally because cheribuild already passes -fuse-ld in the linker flags as
it predates this being done in the makefiles.

Reported By: Jose Luis Duran
Fixes: 172a624f0 ("Silence annoying and incorrect non-default linker warning with GCC")