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
crypto/rsa: fix infinite loop in GenerateMultiPrimeKey for large nprimes

The heuristics for BitLen of a product of randomly generated primes
are wrong, and the generated candidates never match the required
size for nprimes > 10. This corner case is not expected to be used in
practice.

R=agl
CC=golang-dev
https://codereview.appspot.com/7397052