build

Introduce getter functions for host and subsystem objects with on-demand creation #3645

Job	Run time
make static	38s
muon minimal static	34s
nvme-cli (gcc, debug)	49s
cross (armhf)	46s
libnvme (gcc, release)	44s
libnvme (clang, release)	41s
build libnvme and nvme-cli separately	40s
libnvme (gcc, debug)	39s
libnvme (clang, debug)	43s
cross (s390x)	51s
nvme-cli (clang, debug)	50s
nvme-cli (clang, release)	52s
cross (ppc64le)	52s
nvme-cli (gcc, release)	53s
fallback shared libraries	-1s
	10m 31s

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