GC requires the whole zeroed word to be visible for a memory subsystem.
While the implementation of Enhanced REP STOSB tries to use as efficient
stores as possible, e.g writing the whole cache line and not byte-after-byte,
we should use REP STOSQ to guarantee the requirements of the GC.
The performance is not affected.
Change-Id: I1b0fd1444a40bfbb661541291ab96eba11bcc762
Reviewed-on: https://go-review.googlesource.com/c/go/+/405274 Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: Keith Randall <khr@golang.org>