Evaluate gfx_select's #[cfg] conditions at the right time.

[jimblandy]

See the comments in the code for details.

Checklist

• Run cargo clippy.
• Run RUSTFLAGS=--cfg=web_sys_unstable_apis cargo clippy --target wasm32-unknown-unknown if applicable.
• Add change to CHANGELOG.md. See simple instructions inside file.

[codecov-commenter]

Codecov Report

Merging #3253 (f64de4e) into master (46b1216) will increase coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3253      +/-   ##
==========================================
+ Coverage   65.41%   65.60%   +0.19%     
==========================================
  Files          82       82              
  Lines       39455    39468      +13     
==========================================
+ Hits        25809    25894      +85     
+ Misses      13646    13574      -72     

Impacted Files	Coverage Δ
wgpu-core/src/lib.rs	96.55% <ø> (ø)
wgpu-core/src/hub.rs	60.67% <0.00%> (-0.31%)	⬇️
wgpu-core/src/device/queue.rs	70.51% <0.00%> (-0.19%)	⬇️
wgpu-core/src/validation.rs	58.87% <0.00%> (-0.14%)	⬇️
wgpu-core/src/command/clear.rs	89.58% <0.00%> (-0.04%)	⬇️
wgpu-core/src/track/texture.rs	80.97% <0.00%> (-0.02%)	⬇️
wgpu/src/util/mod.rs	12.50% <0.00%> (ø)
wgpu-core/src/instance.rs	65.92% <0.00%> (ø)
wgpu-types/src/lib.rs	88.12% <0.00%> (+0.05%)	⬆️
wgpu-core/src/error.rs	41.60% <0.00%> (+0.72%)	⬆️
... and 3 more

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[cwfitzgerald]

LGTM minus comment