Object: add a comment explaining a divergence

Add a note about the reason for the divergence from the specification
for ld64.  Addresses post-commit review comments from Davide.  NFC.

llvm-svn: 294594
diff --git a/llvm/lib/Object/ArchiveWriter.cpp b/llvm/lib/Object/ArchiveWriter.cpp
index ed4ef6a..4504724 100644
--- a/llvm/lib/Object/ArchiveWriter.cpp
+++ b/llvm/lib/Object/ArchiveWriter.cpp
@@ -316,6 +316,8 @@
   if (HeaderStartOffset == 0)
     return 0;
 
+  // ld64 prefers the cctools type archive which pads its string table to a
+  // boundary of sizeof(int32_t).
   if (Kind == object::Archive::K_BSD)
     for (unsigned P = OffsetToAlignment(NameOS.tell(), sizeof(int32_t)); P--;)
       NameOS << '\0';