linux

rust: allow linking with dynamic libstd #17885

Job	Run time
Arch Linux	20m 23s
Arch / PyPy	21m 56s
CUDA (on Arch)	6m 20s
Fedora	18m 44s
Gentoo	23m 18s
Ubuntu Rolling (gcc, g++)	20m 21s
OpenSUSE	17m 26s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	14m 59s
Ubuntu Bionic	14m 23s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	15m 32s
Ubuntu Rolling (clang, clang++)	23m 35s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	18m 5s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 55s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	14m 0s
	4h 2m 57s

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