Added -float-abi and auto-detection logic for ARM.

Even though this argument design conflates two separate concepts
(ABI and hardware/software implementation), I chose to go
with it since users are liekly know it from GCC and the
combination of softloat operations with hardfloat ABI makes
no sense.

I didn't implement it for old LLVM versions, as ARM EABI
exception handling requires LLVM 3.3+ anyway, without which
LDC would be useless anyway.

driver/target.h

@@ -30,6 +30,15 @@ namespace ExplicitBitness {
     };
 }
 
+namespace FloatABI {
+    enum Type {
+        Default,
+        Soft,
+        SoftFP,
+        Hard
+    };
+}
+
 namespace llvm { class TargetMachine; }
 
 /**
@@ -44,6 +53,7 @@ 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,