[JITLink][arm64] Support arm64e JIT'd code (initially enabled for MachO only).

Adds two new JITLink passes to create and populate a pointer-signing function
that can be called via an allocation-action attached to the LinkGraph:

* createEmptyPointerSigningFunction creates a pointer signing function in a
custome section, reserving sufficient space for the signing code. It should
be run as a post-prune pass (to ensure that memory is reserved prior to
allocation).
* lowerPointer64AuthEdgesToSigningFunction pass populates the signing function
by walking the graph, decoding the ptrauth info (encoded in the edge addend) and
writing an instruction sequence to sign all ptrauth fixup locations.

rdar://61956998