samples/bpf: Enable cross compiler support

When cross compiling, bpf samples use HOSTCC for compiling the non-BPF part of
the sample, however what we really want is to use the cross compiler to build
for the cross target since that is what will load and run the BPF sample.
Detect this and compile samples correctly.
Acked-by: NAlexei Starovoitov <ast@kernel.org>
Signed-off-by: NJoel Fernandes <joelaf@google.com>
Acked-by: NDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

samples/bpf/Makefile

@@ -177,6 +177,11 @@ HOSTLOADLIBES_syscall_tp += -lelf
 LLC ?= llc
 CLANG ?= clang
 
+# Detect that we're cross compiling and use the cross compiler
+ifdef CROSS_COMPILE
+HOSTCC = $(CROSS_COMPILE)gcc
+endif
+
 # Trick to allow make to be run from this directory
 all:
 	$(MAKE) -C ../../ $(CURDIR)/