VFS work (dcache/path walking/mounts) #1111

Job	Run time
build-minimal-sysroots (x86_64)	59s
build-minimal-sysroots (riscv64)	1m 0s
build-minimal-sysroots (arm64)	1m 18s
build-toolchains (ubuntu-latest, x86_64, gnu)	37s
build-toolchains (ubuntu-latest, riscv64, gnu)	36s
build-toolchains (ubuntu-latest, arm64, gnu)	30s
build-toolchains (ubuntu-20.04, x86_64, gnu)	34s
build-toolchains (ubuntu-20.04, riscv64, gnu)	37s
build-toolchains (ubuntu-20.04, arm64, gnu)	26s
build-toolchains (macos-latest, x86_64, gnu)	51s
build-toolchains (macos-latest, riscv64, gnu)	53s
build-toolchains (macos-latest, arm64, gnu)	44s
build-toolchains (ubuntu-latest, llvm, all)	1m 0s
build	2m 46s
build-llvm	3m 11s
build-llvm-arm64	2m 38s
build-arm64	2m 40s
build-riscv	2m 43s
build-llvm-riscv64	2m 33s
run-riscv	0s
run-riscv64-llvm	0s
run-x86_64	0s
run-x86_64-llvm	0s
	26m 36s

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