pipe: implement readv and writev #895

Job	Run time
build-minimal-sysroots (x86_64)	1m 5s
build-minimal-sysroots (riscv64)	1m 14s
build-minimal-sysroots (arm64)	1m 5s
build-toolchains (ubuntu-latest, x86_64, gnu)	34s
build-toolchains (ubuntu-latest, riscv64, gnu)	37s
build-toolchains (ubuntu-latest, arm64, gnu)	26s
build-toolchains (ubuntu-20.04, x86_64, gnu)	52s
build-toolchains (ubuntu-20.04, riscv64, gnu)	54s
build-toolchains (ubuntu-20.04, arm64, gnu)	28s
build-toolchains (macos-latest, x86_64, gnu)	1m 31s
build-toolchains (macos-latest, riscv64, gnu)	1m 27s
build-toolchains (macos-latest, arm64, gnu)	55s
build-toolchains (ubuntu-latest, llvm, all)	1m 37s
build-llvm	2m 53s
build-riscv	2m 29s
build-arm64	2m 51s
build	2m 58s
build-llvm-arm64	2m 30s
build-llvm-riscv64	2m 41s
run-x86_64	0s
run-riscv	0s
run-riscv64-llvm	0s
run-x86_64-llvm	0s
	29m 7s

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