glsl: Add "built-in" functions to do 64x64 => 64 multiplication

glsl: Add "built-in" functions to do 64x64 => 64 multiplication

These functions are directly available in shaders.  A #define is added
to detect the presence.  This allows these functions to be tested using
piglit regardless of whether the driver uses them for lowering.  The
GLSL spec says that functions and macros beginning with __ are reserved
for use by the implementation... hey, that's us!

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>