[WIP] xmp thread safety #1284

Job	Run time
Emscripten	2m 56s
macOS-deps=enabled	40s
macOS-deps=disabled	48s
MSYS2-UCRT64-deps=enabled	1s
MSYS2-CLANG64-deps=enabled	1s
Linux-GCC7-deps=forcefallback	8m 48s
Linux-Clang7-deps=forcefallback	8m 21s
MSYS2-UCRT64-deps=disabled	3m 25s
Linux-GCC7-deps=default	1m 25s
Linux-Clang7-deps=default	1m 43s
MSYS2-CLANG64-deps=disabled	1s
Linux-GCC13-deps=forcefallback	11m 7s
Linux-Clang18-deps=forcefallback	9m 43s
Linux-GCC13-deps=default	4m 15s
Linux-Clang18-deps=default	2m 30s
MSVC-forcefallback-x64	4m 55s
clang-cl-forcefallback	5m 34s
MSVC-forcefallback-x86	6m 0s
clang-cl-default	4m 5s
MSVC-default-x64	2m 24s
MSVC-default-x86	2m 52s
FreeBSD	5m 38s
	1h 27m 12s

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