runtime, cmd/compile: rename memclr -> memclrNoHeapPointers

Since barrier-less memclr is only safe in very narrow circumstances,
this commit renames memclr to avoid accidentally calling memclr on
typed memory. This can cause subtle, non-deterministic bugs, so it's
worth some effort to prevent. In the near term, this will also prevent
bugs creeping in from any concurrent CLs that add calls to memclr; if
this happens, whichever patch hits master second will fail to compile.

This also adds the other new memclr variants to the compiler's
builtin.go to minimize the churn on that binary blob. We'll use these
in future commits.

Updates #17503.

Change-Id: I00eead049f5bd35ca107ea525966831f3d1ed9ca
Reviewed-on: https://go-review.googlesource.com/31369
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/slice.go

@@ -141,7 +141,7 @@ func growslice(et *_type, old slice, cap int) slice {
 		memmove(p, old.array, lenmem)
 		// The append() that calls growslice is going to overwrite from old.len to cap (which will be the new length).
 		// Only clear the part that will not be overwritten.
-		memclr(add(p, newlenmem), capmem-newlenmem)
+		memclrNoHeapPointers(add(p, newlenmem), capmem-newlenmem)
 	} else {
 		// Note: can't use rawmem (which avoids zeroing of memory), because then GC can scan uninitialized memory.
 		p = mallocgc(capmem, et, true)