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

[Brian Cain via llvm-commits]

androm3da 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 [d1f5a9f](https://github.com/llvm/llvm-project/commit/d1f5a9f66ee237eabe35a1adf88eaaaadabd9783), it started causing the test `llvm/test/CodeGen/PowerPC/f128-conv.ll` to fail on multiple bots:

Just based on the files impacted by this commit - it seems like a pretty unlikely cause @dyung .  The commit just prior to that one was 3e2801eb634e6010ee783ea7c93718bbb6ed4c35 and it seems like a *much* better candidate.

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