[JITLink] Update ELF_x86_64 's edge kind to generic edge kind

This patch uses a switch statement to map the ELF_x86_64's edge kind to generic edge kind, and merge the ELF_x86_64 's applyFixup function to the x86_64 's applyFixup function. Some edge kinds were not have corresponding generic edge kinds, so I added three generic edge kinds asa follows:
1. RequestGOTAndTransformToDelta64, which is similar to RequestGOTAndTransformToDelta32.

2. GOTDelta64. This generic kind is similar to Delta64, except the GOTDelta64 computes the delta relative to GOTSymbol

3. RequestGOTAndTransformToGOTDelta64. This edge kind was used to deal with ELF_x86_64's GOT64 edge kind, it request the fixGOTEdge function to change the target to GOT entry, and set the edge kind to generic edge kind GOTDelta64.

These added generic edge kinds may named haphazardly, or can't express its meaning well.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D107967