Fix: use uninitialized LibC::SigsetT (#15144)

`LibC::SigsetT` is a `struct` on some platforms and an alias to `UInt32` on others. `.new` is only valid for the struct variant. `uninitialized` should work in either case.

This code is only used with `-Dgc_none`, so a reproduction needs to include that flag and build for a target where `LibC::SigsetT = alias UInt32`, such as `aarch64-apple-darwin`.

src/crystal/system/unix/pthread.cr

@@ -208,7 +208,7 @@ module Crystal::System::Thread
       Thread.current_thread.@suspended.set(true)
 
       # block all signals but SIG_RESUME
-      mask = LibC::SigsetT.new
+      mask = uninitialized LibC::SigsetT
       LibC.sigfillset(pointerof(mask))
       LibC.sigdelset(pointerof(mask), SIG_RESUME)