compilers/detect: remove unsupported -cpp flag for Clang preprocessor detection #16678

Job	Run time
Arch / PyPy	20m 2s
Arch Linux	20m 16s
Ubuntu Rolling (gcc, g++)	19m 44s
CUDA (on Arch)	7m 48s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 43s
Fedora	20m 2s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	14m 24s
Gentoo	21m 57s
OpenSUSE	16m 22s
Ubuntu Rolling (clang, clang++)	21m 55s
Ubuntu Bionic	15m 26s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 25s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 54s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	14m 17s
	3h 55m 15s

Provide feedback