runtime.wbBufFlush must not modify its arguments, because the
argument slots are also used as spill slots in runtime.gcWriteBarrier.
So, GOEXPERIMENT=clobberdead must not clobber them.

Updates #27326.

Change-Id: Id02bb22a45201eecee748d89e7bdb3df7e4940e4
Reviewed-on: https://go-review.googlesource.com/131957Reviewed-by: Keith Randall <khr@golang.org>