cmd/api: set compiler for all build contexts.
cmd/api: set compiler for all build contexts.

The generated syscall files for Windows are still breaking "go tool api"
(unknown function []byte); I'll look at fixing that separately.

Fixes issue 3285.

R=bradfitz
CC=golang-dev
http://codereview.appspot.com/5777062

tv42/old-go,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,dsriram/golang,tv42/old-go,gnanderson/go,tv42/old-go,tv42/old-go,dsriram/golang,apparentlymart/golang,gnanderson/go,apparentlymart/golang,dsriram/golang,dsriram/golang,gnanderson/go