bpf: Add -target to clang switch while cross compiling.

Update to llvm excludes assembly instructions.
llvm git revision is below

commit 65fad7c26569 ("bpf: add inline-asm support")

This change will be part of llvm  release 6.0

__ASM_SYSREG_H define is not required for native compile.
-target switch includes appropriate target specific files
while cross compiling

Tested on x86 and arm64.

Signed-off-by: Abhijit Ayarekar <abhijit.ayarekar@caviumnetworks.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>