Add FIXMEs about float registers based on section 4.7.

See also
http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20131216/095774.html

Change-Id: Id9757a12acacde2bf96635ff7c085fe195c5fa50
Merge pull request #41 from awong-dev/fixme

Add FIXMEs about float registers based on section 4.7.