[Ada] Fix gmem.out corruption by GNAT.Expect

gcc/ada/

	* adaint.h (__gnat_in_child_after_fork): New flag to express
	child process side after fork call.
	* adaint.c (__gnat_portable_spawn): Set flag
	__gnat_in_child_after_fork.
	* expect.c (__gnat_expect_fork): Set __gnat_in_child_after_fork
	to one on child side.
	* libgnat/memtrack.adb
	(In_Child_After_Fork): Flag to disable memory tracking.
	(Allow_Trace): New routine defining if memory should be tracked.
	(Alloc, Realloc, Free): Use Allow_Trace in "if" condition
	instead of First_Call.