samples/bpf: add back functionality to redefine LLC command

It is practical to be-able-to redefine the location of the LLVM
command 'llc', because not all distros have a LLVM version with bpf
target support.  Thus, it is sometimes required to compile LLVM from
source, and sometimes it is not desired to overwrite the distros
default LLVM version.

This feature was removed with 128d1514 ("samples/bpf: Use llc in
PATH, rather than a hardcoded value").

Add this features back. Note that it is possible to redefine the LLC
on the make command like:

 make samples/bpf/ LLC=~/git/llvm/build/bin/llc

Fixes: 128d1514 ("samples/bpf: Use llc in PATH, rather than a hardcoded value")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>