glsl: Add "built-in" function for 64-bit integer sign()

glsl: Add "built-in" function for 64-bit integer sign()

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>