[BOLT] Fix build-time assertion in RewriteInstance (#90540)

We use pwrite() in RewriteInstance to update contents of existing
sections. pwrite() requires file position to be set past the written
offset which we guarantee at the start of rewriteFile(). Then we had an
implicit assumption in patchBuildID() that the file position will be set
again in patchELFSymTabs() after being reset in patchELFPHDRTable().
That assumption was broken in #90300. The fix is to save and restore
file position in patchELFPHDRTable(). Then we don't have to update it
again in patchELFSymTabs().

Author: maksfb

bolt/lib/Rewrite/RewriteInstance.cpp

@@ -4047,6 +4047,7 @@ void RewriteInstance::patchELFPHDRTable() {
     NewWritableSegmentSize = NextAvailableAddress - NewWritableSegmentAddress;
   }
 
+  const uint64_t SavedPos = OS.tell();
   OS.seek(PHDRTableOffset);
 
   auto createNewTextPhdr = [&]() {
@@ -4151,6 +4152,8 @@ void RewriteInstance::patchELFPHDRTable() {
         << "BOLT-ERROR: could not find PT_GNU_STACK program header to modify\n";
     exit(1);
   }
+
+  OS.seek(SavedPos);
 }
 
 namespace {
@@ -5041,10 +5044,6 @@ void RewriteInstance::patchELFSymTabs(ELFObjectFile<ELFT> *File) {
   assert((DynSymSection || BC->IsStaticExecutable) &&
          "dynamic symbol table expected");
   if (DynSymSection) {
-    // Set pointer to the end of the section, so we can use pwrite to update
-    // the dynamic symbol table.
-    Out->os().seek(DynSymSection->sh_offset + DynSymSection->sh_size);
-
     updateELFSymbolTable(
         File,
         /*IsDynSym=*/true,