cmd/gc, runtime: make assertI2T and variants not variadic

cmd/gc, runtime: make assertI2T and variants not variadic

A side effect of this change is that when assertI2T writes to the
memory for the T being extracted, it can use typedmemmove
for write barriers.

There are other ways we could have done this, but this one
finishes a TODO in package runtime.

Found with GODEBUG=wbshadow=2 mode.
Eventually that will run automatically, but right now
it still detects other missing write barriers.

Change-Id: Icbc8aabfd8a9b1f00be2e421af0e3b29fa54d01e
Reviewed-on: https://go-review.googlesource.com/2279
Reviewed-by: Keith Randall <khr@golang.org>