sched_ext: Improve cross-compilation support in Makefile

sched_ext: Improve cross-compilation support in Makefile

Modify the tools/sched_ext/Makefile to better handle cross-compilation
environments by:

1. Fix host tools build directory structure by separating obj/ from output
   (HOST_BUILD_DIR now points to $(OBJ_DIR)/host/obj)
2. Properly propagate CROSS_COMPILE to libbpf sub-make invocation
3. Add missing $(HOST_BPFOBJ) build rule with proper host toolchain flags
   (ARCH=, CROSS_COMPILE=, explicit HOSTCC/HOSTLD)
4. Consistently quote $(HOSTCC) in bpftool build rule
5. Change LDFLAGS assignment to += to allow external extensions

The changes ensure proper cross-compilation behavior while maintaining
backward compatibility with native builds. Host tools are now correctly
built with the host toolchain while target binaries use the cross-toolchain.

Signed-off-by: yangsonghua <yangsonghua@lixiang.com>
Signed-off-by: Tejun Heo <tj@kernel.org>