MachO: allow each section to have a linker-private symbol

The upcoming ARM64 backend doesn't have section-relative relocations,
so we give each section its own symbol to provide this functionality.
Of course, it doesn't need to appear in the final executable, so
linker-private is the best kind for this purpose.

llvm-svn: 205081
diff --git a/llvm/lib/MC/MCAsmInfo.cpp b/llvm/lib/MC/MCAsmInfo.cpp
index c78d3d5..9667145 100644
--- a/llvm/lib/MC/MCAsmInfo.cpp
+++ b/llvm/lib/MC/MCAsmInfo.cpp
@@ -41,6 +41,7 @@
   LabelSuffix = ":";
   DebugLabelSuffix = ":";
   PrivateGlobalPrefix = "L";
+  LinkerPrivateGlobalPrefix = "";
   InlineAsmStart = "APP";
   InlineAsmEnd = "NO_APP";
   Code16Directive = ".code16";