PR1257 memory barrier

[Joel Sherrill]

Hi,

I have attached the working version of my patch for
the memory barrier discussion.  This part bans
the use of _CPU_ISR_XXXable outside of cpukit.
Use inside cpukit should be in score/cpu.

Chris Johns posted a first version of the patch.
Mine was a manual edit which also covered libcpu,
changed the variable type used as the returned
level and fixed some dreadfully awful spacing
and indentation issues when I was near them.
I tried to avoid getting too sucked into spacing
issues but....

It should all compile.  Instructions for how I
reviewed the patch are included in the PR.

Please review.  I plan to commit this tomorrow.

--joel