Chapter 12 edits

[dankamongmen]

Very few edits here. You suddenly start using MiB in 12-5. I'm all for the MB/MiB distinction, but you should probably use it everywhere if you're going to use it here? I didn't convert MiB to MB (this is your call), but I did change several instances of Mib that clearly meant MiB to MiB.

• 12.1.2: i'd talk about GCC Function Multiversioning (https://gcc.gnu.org/wiki/FunctionMultiVersioning)
• 12.1.3 p208: avoiding FMA might improve performance, but it might hurt accuracy (FMA uses an internal buffer to hold the first result, avoiding the loss of intermediate precision)