Fix: use uninitialized LibC::SigsetT

[straight-shoota]

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.

$ bin/crystal build .test/hello-world.cr -Dgc_none --cross-compile --target=aarch64-apple-darwin24.1.0
Showing last frame. Use --error-trace for full trace.

In src/crystal/system/unix/pthread.cr:211:28

 211 | mask = LibC::SigsetT.new
                            ^--
Error: wrong number of arguments for 'UInt32.new' (given 0, expected 1..7)

Overloads are:
 - UInt32.new(value : String, base : Int = 10, whitespace : Bool = true, underscore : Bool = false, prefix : Bool = false, strict : Bool = true, leading_zero_is_octal : Bool = false)
 - UInt32.new(value)