[ELF] Make compareByFilePosition a strict weak order

This fixes the new test linkerscript/enable-non-contiguous-regions.test
from #90007 in -stdlib=libc++ -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_DEBUG builds.

adjustOutputSections does not discard the output section .potential_a
because it contained .a (which would be spilled to .actual_a).
.potential_a and .bc have the same address and will cause an assertion
failure.

Author: MaskRay

lld/ELF/Writer.cpp

@@ -1338,9 +1338,11 @@ static bool compareByFilePosition(InputSection *a, InputSection *b) {
   OutputSection *aOut = la->getParent();
   OutputSection *bOut = lb->getParent();
 
-  if (aOut != bOut)
-    return aOut->addr < bOut->addr;
-  return la->outSecOff < lb->outSecOff;
+  if (aOut == bOut)
+    return la->outSecOff < lb->outSecOff;
+  if (aOut->addr == bOut->addr)
+    return aOut->sectionIndex < bOut->sectionIndex;
+  return aOut->addr < bOut->addr;
 }
 
 template <class ELFT> void Writer<ELFT>::resolveShfLinkOrder() {