Upgrade bundled mimalloc to 2.1.9 #4422

Job	Run time
build-macos	1m 33s
build-msys	1m 17s
build-freebsd	1m 17s
build-distros (alpine)	1m 20s
build-multi-archs	1m 21s
build-distros (archlinux)	1m 20s
build-sanitizers	1m 31s
build-windows	1m 13s
build-distros (debian:11)	1m 20s
build-sanitizers (-DMOLD_USE_ASAN=On)	1m 30s
build-sanitizers (-DMOLD_USE_TSAN=On)	1m 27s
build-distros (fedora)	1m 21s
build-distros (gentoo/stage3)	1m 24s
build-distros (opensuse/tumbleweed)	1m 19s
build-distros (ubuntu:22.04)	1m 18s
	20m 31s

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