pipe: implement readv and writev #896

Job	Run time
build-minimal-sysroots (x86_64)	49s
build-minimal-sysroots (riscv64)	58s
build-minimal-sysroots (arm64)	2m 28s
build-toolchains (ubuntu-latest, x86_64, gnu)	35s
build-toolchains (ubuntu-latest, riscv64, gnu)	2m 33s
build-toolchains (ubuntu-latest, arm64, gnu)	44s
build-toolchains (ubuntu-20.04, x86_64, gnu)	32s
build-toolchains (ubuntu-20.04, riscv64, gnu)	35s
build-toolchains (ubuntu-20.04, arm64, gnu)	26s
build-toolchains (macos-latest, x86_64, gnu)	1m 21s
build-toolchains (macos-latest, riscv64, gnu)	1m 8s
build-toolchains (macos-latest, arm64, gnu)	2m 14s
build-toolchains (ubuntu-latest, llvm, all)	1m 49s
build	3m 22s
build-llvm	3m 15s
build-llvm-arm64	2m 47s
build-arm64	2m 42s
build-llvm-riscv64	2m 30s
build-riscv	2m 33s
run-x86_64	0s
run-riscv64-llvm	0s
run-riscv	0s
run-x86_64-llvm	0s
	33m 21s

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