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
Sam Clegg