[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 [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 [3e2801e](https://github.com/llvm/llvm-project/commit/3e2801eb634e6010ee783ea7c93718bbb6ed4c35) and it seems like a _much_ better candidate.

Ah indeed, thanks for pointing that out. I didn't see that built by my bot, not sure why though... I'll ping that review.

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