As I mentioned in Optimizing Gentoo: CFLAGS, you want to be comfortable with the existing documentation on CFLAGS before going crazy trying to optimize your CFLAGS on your system. With the wrong CFLAGS things can and will randomly stop working or possibly building. Thus, it's a good idea to have a good understanding of what you're doing to the code when you modify these sacred parameters.
Let's build on what we accomplished last time and get any ebuilds that are machine instruction set aware building those sections as well as the changes you made to the CFLAGS last time.
Finding USE Flag Names
How can we find the flags that we want to add to our /etc/portage/package.use or /etc/make.conf files? As always BASH is our friend and can be used to find this answer for us:
This displays the currently enabled flags based on your CFLAGS parameter from /etc/make.conf and allows us to find which flags have USE flags with the following:
The resulting USE flags can be added to /etc/portage/package.use or /etc/make.conf as your needs dictate.
Using a little scripting we can extract the USE flags that correspond with the instruction sets we enabled last time.