GDB 7.5 recognizes DWARF4 by default.

GDB 7.5 release note does not explicitly mention DWARF4 support
but according to GCC 4.8 release note
 https://gcc.gnu.org/gcc-4.8/changes.html

 "DWARF4 is now the default when generating DWARF debug
 information. ...
 GDB 7.5, Valgrind 3.8.0 and elfutils 0.154 debug information
 consumers support DWARF4 by default."

Change-Id: I56b011c7c38fbc103bbd366ceaea3b709c66ab7f
Reviewed-on: https://go-review.googlesource.com/77570Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>