Add ability to compile program with gprof output

To be able to profile CRIU with gprof support we need a special flag
passed to the compiler and linker. But same time we can't build parasite
code with prof support because it'll call for mcount() service routine
for which we don't have any stub yet (and even if we had there are
other tech details involved). Thus PIE code won't be profiled.

To compile with prof support make it as

  make DEBUG=1 GMON=1

The idea to run gprof then over criu sources. Once you've it
compiled with the flags mentioned above -- simply run it (test or whatever,
I've been running sh test/zdtm.st static.maps00).

Then run gprof ./criu gmon.out

Note the gmon.out file lays in directory where criu were running, thus
in case of sh test/zdtm.st static.maps00 the output file will be in test/
directory.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>