compilersupport_p.h: add a macro for the fallthrough attribute #31

Job	Run time
ubuntu-latest/gcc-no-math	2m 22s
ubuntu-latest/gcc-freestanding	45s
ubuntu-latest/clang	1m 22s
ubuntu-latest/linux-g++	1m 21s
macos-13/clang	1m 38s
	7m 28s

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