remove allows_weak_linkage target spec flag

[RalfJung]

The flag doesn't actually do anything. I have no idea what it's original purpose was.

It got introduced in 59cfe90, but already a few months later in 5795098 both of its uses were removed when the entire file src/librustc_trans/closure.rs got deleted.

It seems like back in the day we used weak linkage for closures by default and MinGW wasn't happy about that? But not much later the way we compile closures got changed, making the work-around unnecessary. The flag has then persisted unused for 10 years. A couple of Windows/UEFI targets are setting allows_weak_linkage: false but I assume that was just cargo-culted.

But to be sure, let's ping the folks listed for the affected targets and other Windows folks:
@dvdhrm @nicholasbishop @Berrysoft @mati865 @thomcc @tbu- @ChrisDenton

[rustbot]

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(See the Target Tier Policy.)

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[mati865]

I wasn't contributing to Rust back when it was added and I have no idea what issue the comment is talking about (I don't deny existence of such bug though).

IIRC, Rust for quite some time now avoids using weak symbols on non-Unix targets.

I'm in favour of removal.