#231 Use libunwind for more accurate callgraph profiling

[Maynard Johnson]

This is a feature request to update operf to use additional information provided by the kernel via the PERF_SAMPLE_STACK_USER sample type. If this sample type is supported (kernel version 3.7?), then operf can pass the additional kernel data to libunwind to accurately ascertain the caller of a function.

[Maynard Johnson]

Use the val4.c testcase to reproduce problems with callgraph which are hinted at in http://oprofile.sourceforge.net/doc/interpreting-callgraph.html. The logic flow in val4.c is:
main -> fn2 -> fn1

Steps to reproduce:
1. On Intel/RHEL 6.4, use stock gcc and compile thusly:
gcc -O1 val4.c -o val4_O1_stock
2. Then run 'operf -g ./val4_O1_stock'
3. Then 'opreport -c' will show an incorrect callgraph:

samples  %        image name               symbol name
-------------------------------------------------------------------------------
27157    49.9706  val4_O1_stock            fn2
  27157    99.8272  val4_O1_stock            fn2 [self]
  47        0.1728  no-vmlinux               /no-vmlinux
-------------------------------------------------------------------------------
27078    49.8252  val4_O1_stock            fn1
  27078    99.8194  val4_O1_stock            fn1 [self]
  49        0.1806  no-vmlinux               /no-vmlinux
-------------------------------------------------------------------------------

^^^^^^^^^^^^^^^

Compiling val4.c with either 'O0' or 'O1 -fno-omit-frame-pointer' seems to resolve or at least ameliorate the problem.