libgomp: Device load_image - improve minor num-funcs/vars check

The run time library loads the offload functions and variable and optionally
the ICV variable and returns the number of loaded items, which has to match
the host side. The plugin returns "+1" (since GCC 12) for the ICV variable
entry, independently whether it was loaded or not, but the var's value
(start == end == 0) can be used to detect when this failed.

Thus, we can tighten the assert check - which this commit does together with
making the output less surprising - and simplify the condition further below.

libgomp/ChangeLog:

	* target.c (gomp_load_image_to_device): Extend fatal-error message;
	simplify a condition.