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
runtime: remove PROT_EXEC from mmap calls.
Executable heap is gone on Unix!

R=golang-dev, dave, bradfitz
CC=golang-dev
https://codereview.appspot.com/7405045