ANDROID: x86: boot: Makefile: pass CLANG_FLAGS to Kbuild

The build machines seem to have older versions of ld.bfd that don't
support R_X86_64_32S absolute relocations, visible as the following
failure in presubmit testing:

Invalid absolute R_X86_64_32S relocation: __end_of_kernel_reserve

It seems that parts of KBUILD that overwrite KBUILD_CFLAGS drop import
clang flags like `--gcc-toolchain` (where to look for the GNU binutils),
so clang just finds the host utilities and just uses those.

This is bad for hermeticity of builds, and needs to be fixed upstream.

Link: https://github.com/ClangBuiltLinux/linux/issues/618


Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I30de5841a50e3fd97ba95c25cc12339d47ad083e