A step towards amaranth `v0.5.0` #2416

Job	Run time
Build regression tests (riscv-tests)	46s
Synthesize full core	32s
Build regression tests (riscv-arch-test)	2m 47s
Check code formatting and typing	46s
Run unit tests	7m 41s
Run regression tests (riscv-tests)	2m 41s
Run regression tests (riscv-arch-test)	6m 37s
	21m 50s

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