Added support for dynamic detection of AVX, and

fixed a few bugs that revealed.  Now the "register
read" command should show AVX registers
(ymm0-ymm15) on Mac OS X platforms that support
them.

When testing this on Mac OS X, run debugserver
manually, like this:

debugserver --native-regs localhost:1111 /path/to/executable

Then

lldb /path/to/executable
...
(lldb) process connect connect://localhost:1111

llvm-svn: 135331