improve cross-compile instructions and makefile support #297

Job	Run time
ci (tpb, ubuntu-22.04, --pb --threads --enable-libffi, test-some-fast)	29m 6s
ci (a6osx, macos-12)	19m 9s
ci (ta6osx, macos-12)	22m 27s
ci (arm64osx, macos-14)	8m 16s
ci (tarm64osx, macos-14)	11m 35s
ci (i3le, ubuntu-22.04)	11m 30s
ci (ti3le, ubuntu-22.04)	14m 17s
ci (a6le, ubuntu-22.04)	8m 59s
ci (ta6le, ubuntu-22.04, CFLAGS+=-std=gnu89)	12m 2s
ci (ta6le, ubuntu-22.04, reboot, -m=ta6le --force, re.boot SCHEME="chezscheme", test-some-fast)	8m 18s
ci (i3nt, windows-2022, vs)	12m 51s
ci (ti3nt, windows-2022, vs)	15m 19s
ci (a6nt, windows-2022, vs)	11m 46s
ci (ta6nt, windows-2022, vs)	14m 33s
ci (ta6nt, windows-2022, gcc, CFLAGS+=-std=gnu89)	13m 41s
	3h 33m 49s

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