adding spirv-std anywhere in the worskspace, slows down the CPU code due to libm feature

[cybersoulK]

rust-gpu/crates/spirv-std/Cargo.toml

Lines 14 to 15 in 1a9c8b8

 num-traits = { version = "0.2.15", default-features = false, features = ["libm"] } 
 glam = { version = ">=0.22, <=0.24", default-features = false, features = ["libm"] } 

it was slowing down my cpu code by 3x, specifically the glam Vec3::distance.
it's really bad because new users might enable it in the shaders crate within the workspace early in development, without realizing the impact in performance.

Possible solution:
spirv-std crate should only enable libm for spirv target in Cargo.toml