os/signal: check int type of signal using reflection

[WineChord]

The current implementation of the signum function only checks if os.Signal is
of type syscall.Signal, then converts it to int. However, golang.org/x/sys
defines its own Signal type for Windows at
https://cs.opensource.google/go/x/sys/+/refs/tags/v0.27.0:windows/syscall_windows.go;l=1472

This causes the signum function and the higher-level signal.Notify function
to fail to recognize signals like windows.SIGINT defined in golang.org/x/sys.

One possible approach is to specifically handle windows.Signal in signum,
but it's unreasonable to make go/src reference golang.org/x/sys.

Another approach is to make windows.Signal directly type alias to
syscall.Signal in golang.org/x/sys, but windows.Signal is fundamentally
different from syscall.Signal (the signals map is different), which is also
not feasible.

Therefore, this PR attempts to use reflection to obtain the possible int
type, to handle this special case for Windows.

Fixes #70369

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Patch Set 1:

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Patch Set 1:

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Patch Set 2:

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Patch Set 5:

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