pprof does not show system functions

Please answer these questions before submitting your issue. Thanks!

### What version of pprof are you using?
If you run pprof from Github, what's the Git revision?
15340c8ac98b453ab008b0744094d10de7bc40d7

### What operating system and processor architecture are you using?
Ubuntu 16.04

### What did you do?
run pprof on the cpu profile output of gperftool library incorporated into c++ binary.

### What did you expect to see?
top shows all the functions (their names).

### What did you see instead?
c++ compiled code indeed showed in "top", however system libraries functions, for example inflateBackEnd from gzip library or __memcpy_avx_unaligned do not show in the output of golang pprof. Instead it's grouped together into a single `` function.
It's showed however in the output of gperftools pprof on the same machine for the same perf data.