[ELF] FIxup buffer pointer before writing synthetic section

[evgeny777]

Summary

I think it makes sense to fix it now, because George (grimar) is preparing a patch allowing to use synthetic sections in linker script.
Normally this is done inside InputSection::writeTo, but synthetic sections were initially output sections and don't do this.

[ruiu]

LGTM. Yeah, this is a big issue if we have multiple synthetic sections in an output section. Thank you for fixing.

[grimar]

Thanks ! :)

ELF/InputSection.cpp

void InputSectionBase<ELFT>::relocate(uint8_t *Buf, uint8_t *BufEnd) {
   }
 }
 
 template <class ELFT> void InputSection<ELFT>::writeTo(uint8_t *Buf) {
   if (this->Type == SHT_NOBITS)
     return;
 
   if (auto *S = dyn_cast<SyntheticSection<ELFT>>(this)) {
-    S->writeTo(Buf);
+    S->writeTo(Buf + OutSecOff);
     return;
   }
 
   // If -r is given, then an InputSection may be a relocation section.
   if (this->Type == SHT_RELA) {
     copyRelocations(Buf + OutSecOff, this->template getDataAs<Elf_Rela>());
     return;
   }