[ELF] Create some synthetic sections only if !relocatable

`.rela.dyn` is currently created outside of the `config->hasDynSymTab`
condition. In relocatable links, `.rela.dyn` will be discarded by
`removeUnusedSyntheticSections`. It's better than suppress the creation
so that .relr.auth.dyn support (#96496) does not need to adjust
`removeUnusedSyntheticSections`.

Author: MaskRay

lld/ELF/SyntheticSections.cpp

@@ -4720,9 +4720,14 @@ template <class ELFT> void elf::createSyntheticSections() {
       add(*part.buildId);
     }
 
+    // dynSymTab is always present to simplify sym->includeInDynsym() in
+    // finalizeSections.
     part.dynStrTab = std::make_unique<StringTableSection>(".dynstr", true);
     part.dynSymTab =
         std::make_unique<SymbolTableSection<ELFT>>(*part.dynStrTab);
+
+    if (config->relocatable)
+      continue;
     part.dynamic = std::make_unique<DynamicSection<ELFT>>();
 
     if (hasMemtag()) {
@@ -4776,19 +4781,17 @@ template <class ELFT> void elf::createSyntheticSections() {
       add(*part.relrDyn);
     }
 
-    if (!config->relocatable) {
-      if (config->ehFrameHdr) {
-        part.ehFrameHdr = std::make_unique<EhFrameHeader>();
-        add(*part.ehFrameHdr);
-      }
-      part.ehFrame = std::make_unique<EhFrameSection>();
-      add(*part.ehFrame);
+    if (config->ehFrameHdr) {
+      part.ehFrameHdr = std::make_unique<EhFrameHeader>();
+      add(*part.ehFrameHdr);
+    }
+    part.ehFrame = std::make_unique<EhFrameSection>();
+    add(*part.ehFrame);
 
-      if (config->emachine == EM_ARM) {
-        // This section replaces all the individual .ARM.exidx InputSections.
-        part.armExidx = std::make_unique<ARMExidxSyntheticSection>();
-        add(*part.armExidx);
-      }
+    if (config->emachine == EM_ARM) {
+      // This section replaces all the individual .ARM.exidx InputSections.
+      part.armExidx = std::make_unique<ARMExidxSyntheticSection>();
+      add(*part.armExidx);
     }
 
     if (!config->packageMetadata.empty()) {

lld/ELF/Writer.cpp

@@ -1458,7 +1458,8 @@ template <class ELFT> void Writer<ELFT>::finalizeAddressDependentContent() {
       in.mipsGot->updateAllocSize();
 
     for (Partition &part : partitions) {
-      changed |= part.relaDyn->updateAllocSize();
+      if (part.relaDyn)
+        changed |= part.relaDyn->updateAllocSize();
       if (part.relrDyn)
         changed |= part.relrDyn->updateAllocSize();
       if (part.memtagGlobalDescriptors)