shims: adjust dispatch shim for LLP64

[compnerd]

Adjust the block type to use intptr_t which is a register-wide integral type.
This allows the same declaration to be portable across LP64 and LLP64
environents.

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[compnerd]

CC: @jrose-apple @DougGregor @gottesmm @MadCoder @ktopley-apple

[compnerd]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 1e89bf0

[compnerd]

@swift-ci please test macOS platform

[jrose-apple]

Seems reasonable to me, although ssize_t would probably be the most appropriate (since the underlying dispatch_apply uses size_t).

[gottesmm]

Whoah its a wild @jrose-apple! Welcome back!

[compnerd]

@jrose-apple - yeah, I can see an argument for that. The reason for me using intptr_t is because this allows us to match the "new" signatures in libdispatch itself. intptr_t is most closely related to the type that they want: something that is pointer width. This means that the type is going to be correct across I64LP32 variants.

[jrose-apple]

Fair enough; the whole point of the trampoline is to get back to Swift.Int.