[BOLT] Set local symbols in relocation mode to zero

Summary:
Strobelight is getting confused by local symbols that we do not
update in relocation mode. These symbols were preserved by the linker in
relocation mode in order support emitting relocations against local
labels, but they are unused.

Issue a quick fix to this by detecting such symbols and setting their
value to zero.

This patch also fixes an issue with the symbol table that was assigning
the wrong section index to symbols associated with the .text section.

(cherry picked from FBD5271277)

bolt/RewriteInstance.cpp

@@ -3000,7 +3000,8 @@ void RewriteInstance::patchELFSectionHeaderTable(ELFObjectFile<ELFT> *File) {
   // Write all section header entries while patching section references.
   for (uint64_t Index = 0; Index < SectionsToWrite.size(); ++Index) {
     auto &Section = SectionsToWrite[Index];
-    if (Section.sh_addr <= NewTextSectionStartAddress &&
+    if (Section.sh_flags & ELF::SHF_ALLOC &&
+        Section.sh_addr <= NewTextSectionStartAddress &&
         Section.sh_addr + Section.sh_size > NewTextSectionStartAddress) {
       NewTextSectionIndex = Index;
     }
@@ -3050,6 +3051,22 @@ void RewriteInstance::patchELFSymTabs(ELFObjectFile<ELFT> *File) {
         if (NewSymbol.st_shndx < ELF::SHN_LORESERVE) {
           NewSymbol.st_shndx = NewSectionIndex[NewSymbol.st_shndx];
         }
+        // Set to zero local syms in the text section that we didn't update
+        // and were preserved by the linker to support relocations against
+        // .text (t15274167).
+        if (opts::Relocs && NewSymbol.getType() == ELF::STT_NOTYPE &&
+            NewSymbol.getBinding() == ELF::STB_LOCAL &&
+            NewSymbol.st_size == 0) {
+          if (auto SecOrErr =
+                  File->getELFFile()->getSection(NewSymbol.st_shndx)) {
+            auto Section = *SecOrErr;
+            if (Section->sh_type == ELF::SHT_PROGBITS &&
+                Section->sh_flags & ELF::SHF_ALLOC &&
+                Section->sh_flags & ELF::SHF_EXECINSTR) {
+              NewSymbol.st_value = 0;
+            }
+          }
+        }
       }
 
       if (opts::HotText) {