libdw is a new dependency built from the elfutils package. It provides
us a way to generate line numbers and file names from the instruction
pointer.

elfutils is LGPLv3 or GPLv2, so it's ok to link against it.

Before:
IGT-Version: 1.23-g8ae86abd419d (x86_64) (Linux: 4.16.0-1-amd64 x86_64)
Starting subtest: fail-result
(meta_test:29661) CRITICAL: Test assertion failure function test_result, file ../tests/meta_test.c:94:
(meta_test:29661) CRITICAL: Failed assertion: result == 1
(meta_test:29661) CRITICAL: error: 0 != 1
Stack trace:
  #0 [__igt_fail_assert+0x20a]
  #1 [test_result+0x7a]
  #2 [__real_main120+0x240]
  #3 [main+0x4a]
  #4 (../csu/libc-start.c) __libc_start_main:344
  #5 [_start+0x2a]

After:
IGT-Version: 1.23-g8ae86abd419d (x86_64) (Linux: 4.16.0-1-amd64 x86_64)
Starting subtest: fail-result
(meta_test:1357) CRITICAL: Test assertion failure function test_result, file ../tests/meta_test.c:94:
(meta_test:1357) CRITICAL: Failed assertion: result == 1
(meta_test:1357) CRITICAL: error: 0 != 1
Stack trace:
  #0 ../lib/igt_core.c:1467 __igt_fail_assert()
  #1 ../tests/meta_test.c:95 test_result()
  #2 ../tests/meta_test.c:137 __real_main120()
  #3 ../tests/meta_test.c:120 main()
  #4 ../csu/libc-start.c:344 __libc_start_main()
  #5

 [_start+0x2a]

Changes since v1:
- Add libdw dependency to readme.
- Change backtrace format slightly.

Cc: Petri Latvala <petri.latvala@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[mlankhorst: Add description about libdw in commit msg, name -> dwfl_name]
Reviewed-by: Petri Latvala <petri.latvala@intel.com>