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

[Ankit Aggarwal via cfe-commits]

aankit-ca wrote:

@alexrp I think this patch is going to have significant impact on code size and performance for Hexagon. Every single access to a global <= 8 will need a constant extender and also consume an extra slot in the packet. Is using the flag directly a viable option for you?

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