cmd/gc: fix method value closures on nacl amd64p32

The code was assuming that pointer alignment is the
maximum alignment, but on NaCl uint64 alignment is
even more strict.

Brad checked in the test earlier today; this fixes the build.

Fixes #7863.

TBR=iant
CC=golang-codereviews
https://golang.org/cl/98630046
2 files changed