[lld][WebAssembly] Allow exporting of mutable globals

In particular allow explict exporting of `__stack_pointer` but
exclud this from `--export-all` to avoid requiring the mutable
globals feature whenenve `--export-all` is used.

This uncovered a bug in populateTargetFeatures regarding checking
if the mutable-globals feature is allowed.

See: https://github.com/WebAssembly/binaryen/issues/2934

Differential Revision: https://reviews.llvm.org/D88506