cmd/dist: avoid using %ebx on i386. Or gcc (-fPIC) will complain: cmd/dist/unix.c: In function ‘cansse2’ cmd/dist/unix.c:774: error: can't find a register in class ‘BREG’ while reloading ‘asm’ cmd/dist/unix.c:774: error: ‘asm’ operand has impossible constraints
cmd/dist: avoid using %ebx on i386.
Or gcc (-fPIC) will complain:
cmd/dist/unix.c: In function ‘cansse2’
cmd/dist/unix.c:774: error: can't find a register in class ‘BREG’ while reloading ‘asm’
cmd/dist/unix.c:774: error: ‘asm’ operand has impossible constraints

This affects bootstrapping on native Darwin/386 where all code is
compiled with -fPIC.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7394047

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