bpf: sockmap sample use clang flag, -target bpf

Per Documentation/bpf/bpf_devel_QA.txt add the -target flag to the
sockmap Makefile. Relevant text quoted here,

   Otherwise, you can use bpf target. Additionally, you _must_ use
   bpf target when:

 - Your program uses data structures with pointer or long / unsigned
   long types that interface with BPF helpers or context data
   structures. Access into these structures is verified by the BPF
   verifier and may result in verification failures if the native
   architecture is not aligned with the BPF architecture, e.g. 64-bit.
   An example of this is BPF_PROG_TYPE_SK_MSG require '-target bpf'

Fixes: 69e8cc134b ("bpf: sockmap sample program")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

samples/sockmap/Makefile

@@ -65,11 +65,14 @@ $(src)/*.c: verify_target_bpf
 # asm/sysreg.h - inline assembly used by it is incompatible with llvm.
 # But, there is no easy way to fix it, so just exclude it since it is
 # useless for BPF samples.
+#
+# -target bpf option required with SK_MSG programs, this is to ensure
+#  reading 'void *' data types for data and data_end are __u64 reads.
 $(obj)/%.o: $(src)/%.c
 	$(CLANG) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(EXTRA_CFLAGS) -I$(obj) \
 		-D__KERNEL__ -D__ASM_SYSREG_H -Wno-unused-value -Wno-pointer-sign \
 		-Wno-compare-distinct-pointer-types \
 		-Wno-gnu-variable-sized-type-not-at-end \
 		-Wno-address-of-packed-member -Wno-tautological-compare \
-		-Wno-unknown-warning-option \
-		-O2 -emit-llvm -c $< -o -| $(LLC) -march=bpf -filetype=obj -o $@
+		-Wno-unknown-warning-option -O2 -target bpf \
+		-emit-llvm -c $< -o -| $(LLC) -march=bpf -filetype=obj -o $@