Fix compiler.preprocess compile_args not used #14492

Job	Run time
Arch Linux	16m 23s
CUDA (on Arch)	5m 35s
Fedora	20m 11s
OpenSUSE	15m 0s
Ubuntu Bionic	14m 8s
Arch / PyPy	19m 0s
Ubuntu Rolling (gcc, g++)	15m 40s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	10m 32s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	10m 59s
Ubuntu Rolling (clang, clang++)	18m 50s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	12m 54s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	11m 22s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	11m 2s
	3h 1m 36s

Provide feedback