Add -float-abi switch and auto-detection for ARM.

This is based on the implementation that was reverted in
fc8e0c4c20.

driver/targetmachine.h

@@ -27,6 +27,15 @@ namespace ExplicitBitness {
     };
 }
 
+namespace FloatABI {
+    enum Type {
+        Default,
+        Soft,
+        SoftFP,
+        Hard
+    };
+}
+
 namespace llvm { class TargetMachine; }
 
 /**
@@ -41,6 +50,7 @@ llvm::TargetMachine* createTargetMachine(
     std::string cpu,
     std::vector<std::string> attrs,
     ExplicitBitness::Type bitness,
+    FloatABI::Type floatABI,
     llvm::Reloc::Model relocModel,
     llvm::CodeModel::Model codeModel,
     llvm::CodeGenOpt::Level codeGenOptLevel,