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

This check is designed to avoid exporting mutable globals in the case
when mutable globals are not available. However, it was being applied in
all cases even when mutable globals was enabled.

This error is particularly bad since mutable-globals have been enabled
in default CPU for a while now, meaning that this condition should not
be firing in the wild very often.

Author: sbc100

lld/test/wasm/mutable-global-exports.s

@@ -73,6 +73,9 @@ _start:
 # CHECK-ALL-NEXT:      - Name:            __wasm_call_ctors
 # CHECK-ALL-NEXT:        Kind:            FUNCTION
 # CHECK-ALL-NEXT:        Index:           0
+# CHECK-ALL-NEXT:      - Name:            __stack_pointer
+# CHECK-ALL-NEXT:        Kind:            GLOBAL
+# CHECK-ALL-NEXT:        Index:           0
 # CHECK-ALL-NEXT:      - Name:            _start
 # CHECK-ALL-NEXT:        Kind:            FUNCTION
 # CHECK-ALL-NEXT:        Index:           1

lld/wasm/Writer.cpp

@@ -784,6 +784,9 @@ void Writer::calculateExports() {
   unsigned globalIndex =
       out.importSec->getNumImportedGlobals() + out.globalSec->numGlobals();
 
+  bool hasMutableGlobals =
+      out.targetFeaturesSec->features.count("mutable-globals") > 0;
+
   for (Symbol *sym : symtab->symbols()) {
     if (!sym->isExported())
       continue;
@@ -801,7 +804,8 @@ void Writer::calculateExports() {
       }
       export_ = {name, WASM_EXTERNAL_FUNCTION, f->getExportedFunctionIndex()};
     } else if (auto *g = dyn_cast<DefinedGlobal>(sym)) {
-      if (g->getGlobalType()->Mutable && !g->getFile() && !g->forceExport) {
+      if (!hasMutableGlobals && g->getGlobalType()->Mutable && !g->getFile() &&
+          !g->isExportedExplicit()) {
         // Avoid exporting mutable globals are linker synthesized (e.g.
         // __stack_pointer or __tls_base) unless they are explicitly exported
         // from the command line.