Add "objects" support to Rust targets #17987

Job	Run time
Arch Linux	20m 42s
CUDA (on Arch)	7m 15s
Arch / PyPy	22m 30s
Ubuntu Rolling (gcc, g++)	22m 20s
Fedora	21m 22s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	15m 6s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	15m 24s
Ubuntu Rolling (clang, clang++)	26m 7s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	18m 21s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 50s
Gentoo	25m 21s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 40s
OpenSUSE	18m 52s
Ubuntu Bionic	16m 12s
	4h 17m 2s

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