selftests/bpf: Add a default $(CXX) value

When using make kselftest TARGETS=bpf, tools/bpf is built with
MAKEFLAGS=rR, which causes $(CXX) to be undefined, which in turn causes
the build to fail with

  CXX      test_cpp
/bin/sh: 2: g: not found

Fix by adding a default $(CXX) value, like tools/build/feature/Makefile
already does.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200602175649.2501580-3-iii@linux.ibm.com

tools/testing/selftests/bpf/Makefile

@@ -2,6 +2,8 @@
 include ../../../../scripts/Kbuild.include
 include ../../../scripts/Makefile.arch
 
+CXX ?= $(CROSS_COMPILE)g++
+
 CURDIR := $(abspath .)
 TOOLSDIR := $(abspath ../../..)
 LIBDIR := $(TOOLSDIR)/lib