Rename K_MIPS64 to K_GNU64

This patch renames K_MIPS64 to K_GNU64 as part of a change to add
support for writing archives with 64-bit indexes in the symbol table.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313787 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed