Rename PltZero -> PltHeader.

PltZero (or PLT[0]) was an appropriate name for the little code
we have at beginning of the PLT section when we only supported x86
since the code for x86 just fits in the first PLT slot.

It's not the case anymore. The code for ARM64 occupies first two
slots, so PltZero spans PLT[0] and PLT[1], for example.
This patch renames it to avoid confusion.

llvm-svn: 272913
diff --git a/lld/ELF/Symbols.cpp b/lld/ELF/Symbols.cpp
index 61f7727..5cbf92f 100644
--- a/lld/ELF/Symbols.cpp
+++ b/lld/ELF/Symbols.cpp
@@ -162,7 +162,7 @@
 }
 
 template <class ELFT> typename ELFT::uint SymbolBody::getPltVA() const {
-  return Out<ELFT>::Plt->getVA() + Target->PltZeroSize +
+  return Out<ELFT>::Plt->getVA() + Target->PltHeaderSize +
          PltIndex * Target->PltEntrySize;
 }