selftests/bpf: fix Makefile for passing LLC to the command line

Makefile has a LLC variable that is initialised to "llc", but can
theoretically be overridden from the command line ("make LLC=llc-6.0").
However, this fails because for LLVM probe check, "llc" is called
directly. Use the $(LLC) variable instead to fix this.

Fixes: 22c88526

 ("bpf: improve selftests and add tests for meta pointer")
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>