[lld] [lld-macho]Define a flag for adjusting slop scale (PR #164295)

[Vy Nguyen via llvm-commits]

oontvoo wrote:

> Can we add a test? Maybe we could have `--slop-scale=1` hit the failure and show that increasing it fixes it.

Well I don't have a small enough test case to check in.
But in principle, this PR isn't changing the thunk allocation algorithm. It technically only makes it a bit configureable. So I think the test should simply be that the new option can be set.


https://github.com/llvm/llvm-project/pull/164295