add comment explaining gcc-shim. (#935) #1025

Job	Run time
Test CUDA support	35s
Test (stable)	28s
Test (beta)	36s
Clippy	14s
Test (nightly)	33s
Test aarch64-apple-tvos	1m 45s
MSRV (ubuntu-latest)	20s
Test (linux32)	48s
MSRV (windows-latest)	37s
Test (macos)	1m 28s
Rustfmt	4s
Test (aarch64-ios)	1m 19s
Test (win32)	1m 22s
Test (win64)	1m 47s
Test (mingw32)	1m 42s
Test (mingw64)	1m 29s
Test (windows-2019)	1m 25s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	2m 27s
	18m 59s

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