dispatch: make public interfaces LLP64 friendly

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This attempts to replace the public uses of unsigned long with
uintptr_t and long with intptr_t. The use of long and unsigned long here was as a type suitable for matching pointer width. However,
on LLP64 targets, this does not hold. Replace them with intptr_t
which gives a proper pointer sized type. This is done only for the
argument types and not for named types to ensure that the ABI is not
modified for C++ (where a template may be specialized on the named
type).

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CC: @parkera @das @MadCoder

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@swift-ci please test

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CC: @ktopley-apple

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@swift-ci please test

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Okay, so, this almost works for everything. The problem becomes dispatch_block_flags_t which is a named type and a typed enumeration. The current enumeration is typedefed to an unsigned long which is not machine word sized on LLP64. Changing the underlying type to uintptr_t would resolve the issue. However, doing so would change the underlying type, which is a problem for C++ code. This would matter specifically where dispatch_block_flags_t is used for a C++ function definition or used in a template specialization, where the change to the underlying type would result in a decoration change and thus break ABI. This obviously would not change the interfaces for libdispatch itself as it is a pure C library. I would love some hints on how to best address that particular type (Is it acceptable to break users in such a way? Or do we want to change the underlying type only on certain targets?)

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Once we know if we can break the ABI or not, we can figure out what to do with #414. If we can break the ABI for C++ users, then that should just work.