[hexagon] Default target for assembler doesn't match compiler

[androm3da]

#125239 changed the compiler driver to target hexagonv68 by default. For the sake of consistency, the assembler and linker (lld) should make a corresponding change.

Also, when making this change please also describe the effects in the 21.0 LLVM release notes.

[llvmbot]

@llvm/issue-subscribers-backend-hexagon

Author: Brian Cain (androm3da)

#125239 changed the compiler driver to target `hexagonv68` by default. For the sake of consistency, the assembler should probably make a corresponding change.

Also, when making this change please also describe the effects in the 21.0 LLVM release notes.