[clang] [llvm] [Hexagon] Set the default compilation target to V68 (PR #125239)

[Brian Cain via cfe-commits]

androm3da wrote:

@quic-akaryaki do we want/need a corresponding change to the assembler?  Or is it okay for the compiler to have a different default cpu from the assembler?

https://github.com/llvm/llvm-project/pull/125239