When SIGCHLD is blocked, some executables (for example CMake) do not exit properly when executed using `SDL_CreateProcess` from any SDL thread (not main thread). `SDL_CreateProcessWithProperties` docs say that `SIGCHILD` should not be ignored or handled, therefore blocking it during thread creation is a likely reason for the bug. Should fix#15210.
Signed-off-by: Konstantin Tomashevich <konstantin.tomashevich@gmail.com>
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