Auto merge of #38146 - kali:master, r=alexcrichton

fix objc ABI in std::env::args

iOS use different calling convention for `objc_msgSend` depending on the platform. armv7 expect good old variadic arguments, but aarch64 wants "normal" convention: `objc_msgSend` has to be called mimicking the actual callee prototype.

https://developer.apple.com/library/content/documentation/General/Conceptual/CocoaTouch64BitGuide/ConvertingYourAppto64-Bit/ConvertingYourAppto64-Bit.html#//apple_ref/doc/uid/TP40013501-CH3-SW26

This currently breaks std::env:args() on aarch64 iOS devices. As far as I can tell, in the standard library, this is the only occurrence of ObjectiveC dispatching.