ANDROID: kbuild: fix build for DEBUG_INFO_BTF_MODULES

Upstream commit f73edc89 ("kbuild: unify two modpost invocations")
adds 'vmlinux' as a Makefile rule dependency for the target 'modules'.
This rule is conditional on DEBUG_INFO_BTF_MODULES like so:

  ifdef CONFIG_DEBUG_INFO_BTF_MODULES
  modules: vmlinux
  endif

However, setting this option breaks android's mixed builds as an
out-of-tree patch skips the definition of the 'vmlinux' rule whenever
KBUILD_MIXED_TREE is set. For details on this approach see commit
ee1b16e8 ("ANDROID: Kbuild: Add support for KBUILD_MIXED_TREE").

Since kbuild is unable to find the 'vmlinux' binary or a rule to
generate it in this case, we hit the following build error:

  $ ./scripts/config -e DEBUG_INFO_BTF
  $ make olddefconfig
  $ KBUILD_MIXED_TREE=${path} make modules
  make[1]: *** No rule to make target 'vmlinux', needed by 'modules'.  Stop.
  make[1]: *** Waiting for unfinished jobs....

In order to fix this issue, lets pass the appropiate path to vmlinux by
prepending mixed-build-prefix. If KBUILD_MIXED_TREE is set, kbuild will
find the previously built 'vmlinux' and won't require a rule for it.

Bug: 266874587
Change-Id: I2290502b557f454187313a563e5477b05d37f3c4
Signed-off-by: Carlos Llamas <cmllamas@google.com>