perf jvmti: unconditionally links against -lelf

[Alexey Brodkin]

Hi Jiri,

Just tried to cross-compile perf with pretty bare-minimal toolchain
which has no libelf installed for the target (for the record I used
SNPS ARC pre-built arc-2016.09 tools) like that:
--------------->8------------
make NO_LIBELF=1
--------------->8------------

And got the following build failure:
--------------->8------------
  LINK     libperf-jvmti.so
.../2016.09-uclibc-archs/bin/../lib/gcc/arc-snps-linux-uclibc/6.2.1/../../../../arc-snps-linux-uclibc/bin/ld: cannot find -lelf
--------------->8------------

First I was surprised because I intentionally disabled usage of libelf
but after some grepping figured out jvmti is linked against libelf
regardless any settings, see:
--------------->8------------
$(OUTPUT)$(LIBJVMTI): $(LIBJVMTI_IN)
       $(QUIET_LINK)$(CC) -shared -Wl,-soname -Wl,$(LIBJVMTI) -o $@ $< -lelf -lrt
endif
--------------->8------------

This comes from commit d4dfdf00d43e ("perf jvmti: Plug compilation into perf build").

I'm not really sure what could be the best solution to the problem I see.
If libelf is a real must for jvmti then probably we may force set NO_JVMTI=1
if NO_LIBELF=1 is passed on the command line.

At least the following builds perf for me without issues:
--------------->8------------
make NO_LIBELF=1 NO_JVMTI=1
--------------->8------------

Regards,
Alexey