[Hexagon] Correct the default compilation target to V68

[iajbar]

Correct the default compilation target to V68 if no Hexagon processor is specified at the command-line.
Add the missing elf header changes for v81/v83/v85 architectures.

[llvmbot]

@llvm/issue-subscribers-bug

Author: Ikhlas Ajbar (iajbar)

Correct the default compilation target to V68 if no Hexagon processor is specified at the command-line. Add the missing elf header changes for v81/v83/v85 architectures.

[llvmbot]

@llvm/issue-subscribers-backend-hexagon

Author: Ikhlas Ajbar (iajbar)

Correct the default compilation target to V68 if no Hexagon processor is specified at the command-line. Add the missing elf header changes for v81/v83/v85 architectures.

[iajbar]

/cherry-pick 759ef58

[llvmbot]

/pull-request #128597