[hexagon] switch to eld as the default linker

[androm3da]

Now that it's available publicly, let's change the compiler driver to pick eld by default. After this change, users can still override with -fuse-ld=lld if they prefer.

[llvmbot]

@llvm/issue-subscribers-backend-hexagon

Author: Brian Cain (androm3da)

Now that it's [available publicly](https://github.com/qualcomm/eld), let's change the compiler driver to pick `eld` by default. After this change, users can still override with `-fuse-ld=lld` if they prefer.

[llvmbot]

@llvm/issue-subscribers-clang-driver

Author: Brian Cain (androm3da)

Now that it's [available publicly](https://github.com/qualcomm/eld), let's change the compiler driver to pick `eld` by default. After this change, users can still override with `-fuse-ld=lld` if they prefer.