[PATCH 0/2][RFC] PPC64 no-exec support

[Jake Moilanen]

These patches add no exec support to PPC64.  It should prohibit
executing code out of the stack, or most any non-text segment.  

For distros that compile w/ pt_gnu_stacks, they depend on Ben's signal
trampoline changes, or else it will hang on the first signal due to the
return code being put on the signal context stack to return to the
kernel on the completion of the signal handler.

The patches include a base fixup from Anton of the wrong bit being used
for no-exec and for read/write on the hardware PTEs.

The patch is broken into two parts:

1/2: PPC64 no-exec support for user space:  This will prohibit user
space apps from executing in segments not marked as executable.  The
base support is in here as well.

2/2: PPC64 no-exec support for kernel space:  This prohibits the kernel
from executing non-text code.

Thanks,
Jake