gl_shader_disk_cache: Special-case boolean handling

Booleans don't have a guaranteed size, but we still want to have them
integrate into the disk cache system without needing to actually use a
different type. We can do this by supplying non-template overloads for
the bool type.

Non-template overloads always have precedence during function
resolution, so this is safe to provide.

This gets rid of the need to smatter ternary conditionals, as well as
the need to use u8 types to store the value in.