[Ada] PR ada/105303 Fix use of Assertion_Policy in internal generics unit

[Ada] PR ada/105303 Fix use of Assertion_Policy in internal generics unit

The internal unit System.Generic_Array_Operations defines only generic
subprograms. Thus, pragma Assertion_Policy inside the spec has no
effect, as each instantiation is only subject to the assertion policy at
the program point of the instantiation. Remove this confusing pragma,
and add the pragma inside each generic body making use of additional
assertions or ghost code, so that running time of instantiations is not
impacted by assertions meant for formal verification.

gcc/ada/

	PR ada/105303
	* libgnat/s-gearop.adb: Add pragma Assertion_Policy in generic
	bodies making use of additional assertions or ghost code.
	* libgnat/s-gearop.ads: Remove confusing Assertion_Policy.