[ELF] Clarify the first entry of .got.plt NFC

[MaskRay]

Summary

glibc sysdeps/x86_64/dl-machine.h (and on some other architectures) uses it to compute run-time load address of the shared object.

ELF/Arch/X86_64.cpp

RelExpr X86_64<ELFT>::getRelExpr(RelType Type, const Symbol &S,
   case R_X86_64_NONE:
     return R_NONE;
   default:
     return R_INVALID;
   }
 }
 
 template <class ELFT> void X86_64<ELFT>::writeGotPltHeader(uint8_t *Buf) const {
-  // The first entry holds the value of _DYNAMIC. It is not clear why that is
-  // required, but it is documented in the psabi and the glibc dynamic linker
-  // seems to use it (note that this is relevant for linking ld.so, not any
-  // other program).
+  // The first entry holds the link-time address of _DYNAMIC. It is documented
+  // in the psABI and glibc dynamic linker elf_machine_dynamic uses it to
+  // compute run-time load address of the shared object (note that this is
+  // relevant for linking ld.so, not any other program).
   write64le(Buf, InX::Dynamic->getVA());
 }
 
 template <class ELFT>
 void X86_64<ELFT>::writeGotPlt(uint8_t *Buf, const Symbol &S) const {
   // See comments in X86::writeGotPlt.
   write32le(Buf, S.getPltVA() + 6);
 }