Implement target independent TLS compatible with glibc's emutls.c.

The 'common' section TLS is not implemented.
Current C/C++ TLS variables are not placed in common section.
DWARF debug info to get the address of TLS variables is not generated yet.

clang and driver changes in http://reviews.llvm.org/D10524

  Added -femulated-tls flag to select the emulated TLS model,
  which will be used for old targets like Android that do not
  support ELF TLS models.

Added TargetLowering::LowerToTLSEmulatedModel as a target-independent
function to convert a SDNode of TLS variable address to a function call
to __emutls_get_address.

Added into lib/Target/*/*ISelLowering.cpp to call LowerToTLSEmulatedModel
for TLSModel::Emulated. Although all targets supporting ELF TLS models are
enhanced, emulated TLS model has been tested only for Android ELF targets.
Modified AsmPrinter.cpp to print the emutls_v.* and emutls_t.* variables for
emulated TLS variables.
Modified DwarfCompileUnit.cpp to skip some DIE for emulated TLS variabls.

TODO: Add proper DIE for emulated TLS variables.
      Added new unit tests with emulated TLS.

Differential Revision: http://reviews.llvm.org/D10522


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243438 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/AsmPrinter.h

@@ -238,6 +238,11 @@ public:
   ///
   void EmitJumpTableInfo();
 
+  /// Emit the control variable for an emulated TLS variable.
+  virtual void EmitEmulatedTLSControlVariable(const GlobalVariable *GV,
+                                              MCSymbol *EmittedSym,
+                                              bool AllZeroInitValue);
+
   /// Emit the specified global variable to the .s file.
   virtual void EmitGlobalVariable(const GlobalVariable *GV);