[clang] [llvm] [Driver] Default -msmall-data-limit= to 0 and clean up code (PR #83093)

[Sam Elliott via cfe-commits]

lenary wrote:

I was a reviewer on the original patch, and I can see that I let some not great things through - claiming options that should not have been claimed, etc.

I also do think that the situation has changed since then - more platform defaults are opting out of the sbss/sdata optimisation than opting into it, and the GP relaxation is now off-by-default in LLD.

I'm not sure how the default was chosen, or how representative it is, so I'm happy with this, as any toolchains can use configuration files for e.g. baremetal configurations. A lot of choice around the "right" value is going to heavily depend on the specific application, I expect (and probably also the word size, which hasn't been taken into account by this default anyway).

https://github.com/llvm/llvm-project/pull/83093