build: downgrade GCC support to experimental #4944

Job	Run time
Build slim tarball	2m 47s
x86_64-linux: with shared libraries	1h 49m 11s
aarch64-linux: with shared libraries	1h 19m 33s
x86_64-darwin: with shared libraries	2h 32m 21s
aarch64-darwin: with shared libraries	1h 48m 15s
	7h 32m 7s

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