selftests: use "$(MAKE)" instead of "make"

When doing "make kselftest TARGETS=bpf -j12", bpf progs end up being
compiled sequentially and thus slowly.

The reason is that parent make (tools/testing/selftests/Makefile) does
not share its jobserver with child make
(tools/testing/selftests/bpf/Makefile), therefore the latter runs with
-j1.

Change all instances of "make" to "$(MAKE)", so that the whole make
hierarchy runs using a single jobserver.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>