MSVC: Add support for linking against the "spectre-mitigated" CRT #1434

Job	Run time
Test (stable)	47s
Test CUDA support	35s
MSRV (ubuntu-latest)	23s
Test (beta)	1m 18s
Clippy	7s
MSRV (windows-latest)	42s
Test (nightly)	1m 17s
Rustfmt	9s
Test (linux32)	1m 18s
Test (macos)	2m 52s
Test (aarch64-macos)	1m 1s
Test build-std (aarch64-tvos)	1m 48s
Test (aarch64-ios)	1m 17s
Test build-std (aarch64-tvos-sim)	2m 34s
Test (win32)	2m 22s
Test build-std (x86_64-tvos)	1m 51s
Test (win64)	3m 8s
Test (mingw32)	2m 26s
Test (mingw64)	2m 3s
Test (windows-2019)	4m 26s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	3m 18s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	3m 57s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	4m 4s
	43m 43s

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