kbuild: change tool coverage variables to take the path relative to $(obj)

Commit 54b8ae66 ("kbuild: change *FLAGS_<basetarget>.o to take the
path relative to $(obj)") changed the syntax of per-file compiler flags.

The situation is the same for the following variables:

  OBJECT_FILES_NON_STANDARD_<basetarget>.o
  GCOV_PROFILE_<basetarget>.o
  KASAN_SANITIZE_<basetarget>.o
  KMSAN_SANITIZE_<basetarget>.o
  KMSAN_ENABLE_CHECKS_<basetarget>.o
  UBSAN_SANITIZE_<basetarget>.o
  KCOV_INSTRUMENT_<basetarget>.o
  KCSAN_SANITIZE_<basetarget>.o
  KCSAN_INSTRUMENT_BARRIERS_<basetarget>.o

The <basetarget> is the filename of the target with its directory and
suffix stripped.

This syntax comes into a trouble when two files with the same basename
appear in one Makefile, for example:

  obj-y += dir1/foo.o
  obj-y += dir2/foo.o
  OBJECT_FILES_NON_STANDARD_foo.o := y

OBJECT_FILES_NON_STANDARD_foo.o is applied to both dir1/foo.o and
dir2/foo.o. This syntax is not flexbile enough to handle cases where
one of them is a standard object, but the other is not.

It is more sensible to use the relative path to the Makefile, like this:

  obj-y += dir1/foo.o
  OBJECT_FILES_NON_STANDARD_dir1/foo.o := y
  obj-y += dir2/foo.o
  OBJECT_FILES_NON_STANDARD_dir2/foo.o := y

To maintain the current behavior, I made adjustments to the following two
Makefiles:

 - arch/x86/entry/vdso/Makefile, which compiles vclock_gettime.o, vgetcpu.o,
   and their vdso32 variants.

 - arch/x86/kvm/Makefile, which compiles vmx/vmenter.o and svm/vmenter.o

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Acked-by: Sean Christopherson <seanjc@google.com>