[clang] [llvm] [Hexagon] Set `hexagon-small-data-threshold` to zero by default (PR #187344)

[Brian Cain via cfe-commits]

androm3da wrote:

I'm going to defer to @aankit-ca though I suppose Ankit won't have enough history/context to know why it was chosen the way it was?

I would hope that either @SundeepKushwaha or @quic-seaswara will know why we want small data on by default, because I don't.   I suspect it gives better code size performance on benchmarks and that's why?

> it depends on linker support which LLD does not have

I'll keep fixing hexagon lld bugs as we encounter them.  But it's worth mentioning that the vast majority of hexagon codegen put to use in real devices gets linked with `ld.eld`.

I will concede that I like the uniformity when the llvm project comes with its own linker.  Though, at the same time, there's some [risk of a monoculture there too](https://lists.llvm.org/pipermail/llvm-dev/2019-June/133308.html).

https://github.com/llvm/llvm-project/pull/187344