[lld] [llvm] [hexagon] Bump the default version to v68 (PR #132304)

[via llvm-commits]

dyung wrote:

Hi @quic-akaryaki and @tstellar, not sure if you are aware, but when this change was cherry-picked to the `release/20.x` branch in d1f5a9f66ee237eabe35a1adf88eaaaadabd9783, it started causing the test `llvm/test/CodeGen/PowerPC/f128-conv.ll` to fail on multiple bots:
- https://lab.llvm.org/buildbot/#/builders/115/builds/86
- https://lab.llvm.org/buildbot/#/builders/62/builds/120
- https://lab.llvm.org/buildbot/#/builders/156/builds/71
- https://lab.llvm.org/buildbot/#/builders/31/builds/113
- https://lab.llvm.org/buildbot/#/builders/100/builds/133
- https://lab.llvm.org/buildbot/#/builders/102/builds/163

https://github.com/llvm/llvm-project/pull/132304