ada: Remove useless trampolines caused by Unchecked_Conversion

ada: Remove useless trampolines caused by Unchecked_Conversion

The partial solution implemented in Validate_Unchecked_Conversion to support
unchecked conversions between addresses and pointers to subprograms, for the
platforms where pointers to subprograms do not all have the same size, turns
out to be counter-productive for others because it may cause the creation of
useless trampolines, which in turn makes the stack executable.

gcc/ada/

	* sem_ch13.adb (Validate_Unchecked_Conversion): Restrict forcing the
	Can_Use_Internal_Rep flag to platforms that require unnesting.