[lld] [lld][Hexagon] Add "hexagonlinux" emulation alias (PR #185743)

[Brian Cain via llvm-commits]

androm3da wrote:

> We actively discourage unnecessary emulations and linker-based discrepancies. Instead, we advocate for passing explicit linker options directly through the Clang driver. In addition, I don't see "hexagonlinux" in llvm-project yet.

I can land #185745 first before this one if you prefer, @MaskRay

I intend for this emulation to be used in compatibility with `ld.eld`'s emulation -- apparently it does need this distinction https://github.com/qualcomm/eld/pull/932


https://github.com/llvm/llvm-project/pull/185743