ARM IAS: support .object_arch

The .object_arch directive indicates an alternative architecture to be specified
in the object file.  The directive does *not* effect the enabled feature bits
for the object file generation.  This is particularly useful when the code
performs runtime detection and would like to indicate a lower architecture as
the requirements than the actual instructions used.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200451 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCStreamer.h

@@ -107,6 +107,7 @@ public:
                                     StringRef StringValue = "") = 0;
   virtual void emitFPU(unsigned FPU) = 0;
   virtual void emitArch(unsigned Arch) = 0;
+  virtual void emitObjectArch(unsigned Arch) = 0;
   virtual void finishAttributeSection() = 0;
   virtual void emitInst(uint32_t Inst, char Suffix = '\0') = 0;