make.bash: detect and warn about SELinux policy that crashes us.
The default SELinux policy on Fedora 12 (at least) disallows stack
pages to be +x. This causes all binaries written by 6g/6l to segfault
immedately. The 'true' way to fix this issue is to mark binaries with
the correct type. However, that assumes that 6l is going to detect
SELinux, figure out the correct type for the current distribution and
set the type (without libselinux).
For now we'll warn users and point them towards the way to enable
execstack for the whole system.
http://code.google.com/p/go/issues/detail?id=47
R=rsc
CC=golang-dev
http://codereview.prom.corp.google.com/
1026041