glsl: Define isnormal and copysign for MSVC to fix build.

glsl: Define isnormal and copysign for MSVC to fix build.

This patch fixes these MSVC build errors.

ir_constant_expression.cpp
src\glsl\ir_constant_expression.cpp(564) : warning C4244: '=' : conversion from 'int' to 'float', possible loss of data
src\glsl\ir_constant_expression.cpp(1384) : error C3861: 'isnormal': identifier not found
src\glsl\ir_constant_expression.cpp(1385) : error C3861: 'copysign': identifier not found

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=69541
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Acked-by: Matt Turner <mattst88@gmail.com>