piegen: Allow to compile piegen tool in cross environment

To build piegen tool with different compiler/linker than
gcc/ld -- simply run make as

	HOSTCC="host-compiler" HOSTLD="host-ld" make

where host-compiler/ld is appropriate program needed.

https://github.com/xemul/criu/issues/63



Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Reviewed-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>