Emscripten: re-enable the -pthread compiler flag #12945

Job	Run time
Code style check / Code style check	6s
CodeChecker / CodeChecker	29m 39s
IWYU / IWYU	3m 13s
SonarCloud Analyzer / SonarCloud Analyzer	0s
MSVC / MSVC (SDL2 x86)	4m 29s
MSVC / MSVC (SDL2 x64)	4m 29s
Android / Android	9m 45s
CMake / CMake (Windows SDL2)	5m 10s
CMake / CMake (Linux SDL2)	3m 5s
CMake / CMake (macOS SDL2)	1m 53s
Make / Make (Linux x86-64 SDL2 Release)	2m 55s
CMake / CMake (macOS SDL2 App Bundle)	2m 13s
Make / Make (Emscripten)	3m 52s
Make / Make (Linux x86-64 SDL2 Debug)	3m 16s
Make / Make (PS Vita)	9m 18s
Make / Make (Linux ARM64 SDL2 Release)	4m 51s
Make / Make (Nintendo Switch)	3m 6s
Make / Make (Linux ARM64 SDL2 Debug)	4m 28s
Make / Make (macOS SDL2)	4m 6s
Make / Make (macOS SDL2 App Bundle)	1m 40s
	1h 41m 34s

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