runtime: simplify mallocgc flag argument

runtime: simplify mallocgc flag argument

mallocgc can calculate noscan itself.  The only remaining
flag argument is needzero, so we just make that a boolean arg.

Fixes #15379

Change-Id: I839a70790b2a0c9dbcee2600052bfbd6c8148e20
Reviewed-on: https://go-review.googlesource.com/22290
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>