build: rework coverage and ubsan CFLAGS handling

build: rework coverage and ubsan CFLAGS handling

When assigning a value a target-specific variable, that also affect
prerequisite of the target. This is mostly fine, but there is one case
where we will not want the COV_FLAGS added to the CFLAGS.

In arch/x86/boot, we have "head.o" with "cmdline.S" as prerequisite
and ultimately "cmdline.o", we don't want COV_FLAGS to that last one.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>