Minor changes in x86 intrinsics headers; NFC

I made several changes for consistency with the rest of x86 instrinsics header files. Some of these changes help to render doxygen comments better.

1. avxintrin.h –  Moved the opening bracket on a separate line for several 
                  intrinsics (for consistency with the rest of the intrinsics).

2. emmintrin.h -  Moved the doxygen comment next to the body of the function;
               -  Added braces after extern "C"  even though there is only 
                  one declaration each time

3. xmmintrin.h -  Moved the doxygen comment next to the body of the function;
               -  Added intrinsic prototypes for a couple of macro definitions
                  into the doxygen comment;
               -  Added braces after extern "C"  even though there is only one
                  declaration each time

4. ammintrin.h –  Removed extra line between the doxygen comment and the body
                  of the functions (for consistency with the rest of the files).

Desk reviewed by Paul Robinson.

llvm-svn: 287278