[ELF][PPC64] Fix potentially corrupted section content with empty .got

D91426 makes .got possibly empty while needed. If .got and .data have the same
address, and .got's content is written after .data, the first word of .data will
be corrupted.

The bug is not testable without D131247.

(cherry picked from commit 28d05d6)

Author: MaskRay

lld/ELF/SyntheticSections.cpp

@@ -712,6 +712,9 @@ bool GotSection::isNeeded() const {
 }
 
 void GotSection::writeTo(uint8_t *buf) {
+  // On PPC64 .got may be needed but empty. Skip the write.
+  if (size == 0)
+    return;
   target->writeGotHeader(buf);
   relocateAlloc(buf, buf + size);
 }