scripts/gen-btf.sh: Fix .btf.o generation when compiling for RISCV

scripts/gen-btf.sh: Fix .btf.o generation when compiling for RISCV

gen-btf.sh emits a .btf.o file with BTF sections to be linked into
vmlinux in link-vmlinux.sh

This .btf.o file is created by compiling an emptystring with ${CC},
and then adding BTF sections into it with ${OBJCOPY}.

To ensure the .btf.o is linkable when cross-compiling with LLVM, we
have to also pass ${KBUILD_FLAGS}, which in particular control the
target word size.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202512240559.2M06DSX7-lkp@intel.com/
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20251229202823.569619-1-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>