Switch krait to use -mcpu=cortex-a15 for assembler tool invocations.

[srhines]

Summary

Using -no-integrated-as causes -mcpu=krait to be transformed into
-march=armv7-a today. This precludes the assembler from using
instructions like sdiv, which are present for krait. Cortex-a15 is the
closest subset of functionality for krait, so we should switch the
assembler to use that instead.

[weimingz]

LGTM. Ana, could you take a look too?

[weimingz]

Ana is OK with it.

lib/Driver/Tools.cpp

case llvm::Triple::thumbeb: {
     case arm::FloatABI::Hard:
       CmdArgs.push_back(Args.MakeArgString("-mfloat-abi=hard"));
       break;
     }
 
     Args.AddLastArg(CmdArgs, options::OPT_march_EQ);
 
     // FIXME: remove krait check when GNU tools support krait cpu
-    // for now replace it with -march=armv7-a  to avoid a lower
+    // for now replace it with -mcpu=cortex-a15 to avoid a lower
     // march from being picked in the absence of a cpu flag.
     Arg *A;
     if ((A = Args.getLastArg(options::OPT_mcpu_EQ)) &&
         StringRef(A->getValue()).lower() == "krait")
-      CmdArgs.push_back("-march=armv7-a");
+      CmdArgs.push_back("-mcpu=cortex-a15");
     else
       Args.AddLastArg(CmdArgs, options::OPT_mcpu_EQ);
     Args.AddLastArg(CmdArgs, options::OPT_mfpu_EQ);
     break;
   }
   case llvm::Triple::mips:
   case llvm::Triple::mipsel:
   case llvm::Triple::mips64: