pipe: implement writev #898

Job	Run time
build-minimal-sysroots (x86_64)	53s
build-minimal-sysroots (riscv64)	58s
build-minimal-sysroots (arm64)	1m 4s
build-toolchains (ubuntu-latest, x86_64, gnu)	41s
build-toolchains (ubuntu-latest, riscv64, gnu)	53s
build-toolchains (ubuntu-latest, arm64, gnu)	44s
build-toolchains (ubuntu-20.04, x86_64, gnu)	32s
build-toolchains (ubuntu-20.04, riscv64, gnu)	34s
build-toolchains (ubuntu-20.04, arm64, gnu)	43s
build-toolchains (macos-latest, x86_64, gnu)	1m 27s
build-toolchains (macos-latest, riscv64, gnu)	1m 3s
build-toolchains (macos-latest, arm64, gnu)	1m 2s
build-toolchains (ubuntu-latest, llvm, all)	1m 29s
build-llvm-riscv64	2m 33s
build-riscv	2m 25s
build	3m 2s
build-arm64	2m 44s
build-llvm	2m 49s
build-llvm-arm64	2m 49s
run-riscv	0s
run-x86_64	0s
run-riscv64-llvm	0s
run-x86_64-llvm	0s
	28m 25s

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