Make IO asynchronous #889

Job	Run time
build-minimal-sysroots (x86_64)	48s
build-minimal-sysroots (riscv64)	1m 32s
build-minimal-sysroots (arm64)	49s
build-toolchains (ubuntu-latest, x86_64, gnu)	17m 31s
build-toolchains (ubuntu-latest, riscv64, gnu)	12m 15s
build-toolchains (ubuntu-latest, arm64, gnu)	13m 30s
build-toolchains (ubuntu-20.04, x86_64, gnu)	18m 36s
build-toolchains (ubuntu-20.04, riscv64, gnu)	13m 26s
build-toolchains (ubuntu-20.04, arm64, gnu)	15m 38s
build-toolchains (macos-latest, x86_64, gnu)	30m 37s
build-toolchains (macos-latest, riscv64, gnu)	22m 47s
build-toolchains (macos-latest, arm64, gnu)	1m 10s
build-toolchains (ubuntu-latest, llvm, all)	1h 49m 44s
build-riscv	2m 45s
build	3m 0s
build-llvm	2m 50s
build-arm64	3m 3s
build-llvm-arm64	2m 23s
build-llvm-riscv64	2m 29s
run-riscv	0s
run-x86_64	0s
run-x86_64-llvm	0s
run-riscv64-llvm	0s
	4h 34m 53s

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