VFS work (dcache/path walking/mounts) #1114

Job	Run time
build-minimal-sysroots (x86_64)	1m 10s
build-minimal-sysroots (riscv64)	1m 2s
build-minimal-sysroots (arm64)	1m 11s
build-toolchains (ubuntu-latest, x86_64, gnu)	32s
build-toolchains (ubuntu-latest, riscv64, gnu)	42s
build-toolchains (ubuntu-latest, arm64, gnu)	29s
build-toolchains (ubuntu-20.04, x86_64, gnu)	33s
build-toolchains (ubuntu-20.04, riscv64, gnu)	38s
build-toolchains (ubuntu-20.04, arm64, gnu)	26s
build-toolchains (macos-latest, x86_64, gnu)	46s
build-toolchains (macos-latest, riscv64, gnu)	49s
build-toolchains (macos-latest, arm64, gnu)	38s
build-toolchains (ubuntu-latest, llvm, all)	1m 12s
build-llvm	1m 12s
build	1m 17s
build-arm64	1m 28s
build-llvm-arm64	1m 37s
build-riscv	1m 8s
build-llvm-riscv64	1m 32s
run-riscv	0s
run-riscv64-llvm	0s
run-x86_64	0s
run-x86_64-llvm	0s
	18m 22s

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