gn: Optionally disable optimizations and HLSL

gn: Optionally disable optimizations and HLSL

To reduce the binary size of ANGLE, a gn override is added
(glslang_angle) which:

- Controls whether ENABLE_OPT=1 is set
- Customizes the build for the Vulkan backend of ANGLE.  As a first
  step, this removes HLSL functionality which together with no
  optimization shave ~2.5MB off of ANGLE's binary size.

Upcoming changes will add a macro for GLSLANG_ANGLE similar to
GLSLANG_WEB that will strip features from glslang to support only what
ANGLE needs.

Signed-off-by: Shahbaz Youssefi <ShabbyX@gmail.com>