Memory (Linux): Use `MemAvailable` if available #504

Job	Run time
Linux	3m 59s
FreeBSD	9m 43s
MacOS	12m 45s
Linux build test (ubuntu-22.04, gcc, g++, ON)	55s
spellcheck	12s
Linux build test (ubuntu-22.04, gcc, g++, OFF)	1m 14s
Win32	11m 22s
Win64	4m 41s
Linux build test (ubuntu-22.04, clang, clang++, ON)	1m 34s
Linux build test (ubuntu-22.04, clang, clang++, OFF)	1m 11s
Linux build test (ubuntu-22.04, musl-gcc, musl-g++, OFF)	1m 7s
Linux build test (ubuntu-20.04, gcc, g++, ON)	1m 51s
Linux build test (ubuntu-20.04, gcc, g++, OFF)	1m 11s
Linux build test (ubuntu-20.04, clang, clang++, ON)	1m 28s
Linux build test (ubuntu-20.04, clang, clang++, OFF)	1m 7s
Linux build test (ubuntu-20.04, musl-gcc, musl-g++, OFF)	1m 16s
Release	0s
	55m 36s

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