runtime/internal/atomic: remove bad go:noescape annotations on Loadp
The //go:noescape directive says that arguments don't leak at all,
which is too aggressive of a claim for functions that return pointers
derived from their parameters.
Remove the directive for now. Long term fix will require a new
directive that allows more fine-grained control over escape analysis
information supplied for functions implemented in assembly.
Also, update the BAD comments in the test cases for Loadp: we really
want that *ptr leaks to the result parameter, not that *ptr leaks to
the heap.