As a side effect of working on mid-stack inlining, we've fixed support
for inlining variadic functions. Might as well enable it.

Change-Id: I7f555f8b941969791db7eb598c0b49f6dc0820aa
Reviewed-on: https://go-review.googlesource.com/100456Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>