[lld] [lld][WebAssembly] Fix check for implicitly exported mutable globals (PR #160966)

[Sam Clegg via llvm-commits]

sbc100 wrote:

> So before we were basically just never exporting the stack pointer when EXPORT_ALL was set, and now we should be when mutable globals is available?

Yes, exactly.  

https://github.com/llvm/llvm-project/pull/160966