bpf-headers: fix llvm invocation

- specify CC target explicitly
- define KBUILD_SYM32=n to select "native" build format (if building with 64-bit arch)
- rewrap lines (for good)

Signed-off-by: Konstantin Demin <rockdrilla@gmail.com>
Link: openwrt#15702
Signed-off-by: Robert Marko <robimarko@gmail.com>

package/kernel/bpf-headers/Makefile

@@ -53,7 +53,9 @@ KERNEL_MAKE := \
 	$(MAKE) -C $(PKG_BUILD_DIR) \
 		ARCH=$(BPF_KARCH) \
 		CROSS_COMPILE=$(BPF_ARCH)-linux- \
-		LLVM=1 CC="$(CLANG)" LD="$(TARGET_CROSS)ld" \
+		LLVM=1 KBUILD_SYM32=n \
+		CC="$(CLANG) --target=$(BPF_TARGET)" \
+		LD="$(TARGET_CROSS)ld" \
 		HOSTCC="$(HOSTCC)" \
 		HOSTCXX="$(HOSTCXX)" \
 		KBUILD_HOSTLDLIBS="-L$(STAGING_DIR_HOST)/lib" \