cmd/8g: out of fixed registers

<pre>go version devel +80c5e57e2ec3 Mon May 19 12:06:30 2014 +0400 + linux/amd64

Program is:
<a href="http://play.golang.org/p/QUO0TkWevZ">http://play.golang.org/p/QUO0TkWevZ</a>

$ GOARCH=386 go build src.go
registers allocated at
    AX  0
    CX  0
    DX  0
    BX  0x4ce0fd
    SP  0
    BP  0x4ca962
    SI  0x4cd431
    DI  0x4cffab
src.go:31: out of fixed registers</pre>
