Newer GCC Function Attributes

[Joel Sherrill]

Hi

It looks like GCC has gotten some pretty nifty attributes for arguments and
return values.

https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html

You can specify allowed access mode for non-const pointers abd lignment
hints for returned pointers.

I'm not sure if we have any good use cases for these but wanted to toss
this out there in case someone thinks of something I didn't.

--joel