Add entry point attribute configuration.

[XAMPPRocky]

Adds an nested entry_point attribute to all the execution models attributes. This allows you to override the OpEntryPoint for a given entry point with your own name.

Example

#[allow(unused_attributes)]
#[spirv(fragment(entry_point_name="hello_world"))]
pub fn main() { }

[eddyb]

You need to remove #[allow(unused_attributes)] from the PR description and the test, and rebase past #461.