samples/bpf: Use --target from cross-compile

For cross compiling the target triple can be inherited from
cross-compile prefix as it's done in CLANG_FLAGS from kernel makefile.
So copy-paste this decision from kernel Makefile.
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20191011002808.28206-4-ivan.khoronzhuk@linaro.org

samples/bpf/Makefile

@@ -195,7 +195,7 @@ BTF_PAHOLE ?= pahole
 # Detect that we're cross compiling and use the cross compiler
 ifdef CROSS_COMPILE
 HOSTCC = $(CROSS_COMPILE)gcc
-CLANG_ARCH_ARGS = -target $(ARCH)
+CLANG_ARCH_ARGS = --target=$(notdir $(CROSS_COMPILE:%-=%))
 endif
 
 # Don't evaluate probes and warnings if we need to run make recursively