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/main.cpp

@@ -459,8 +459,9 @@ int main(int argc, char** argv)
     if (global.errors)
         fatal();
 
-    gTargetMachine = createTargetMachine(mTargetTriple, mArch, mCPU, mAttrs, bitness,
-        mRelocModel, mCodeModel, codeGenOptLevel(), global.params.symdebug);
+    gTargetMachine = createTargetMachine(mTargetTriple, mArch, mCPU, mAttrs,
+        bitness, mFloatABI, mRelocModel, mCodeModel, codeGenOptLevel(),
+        global.params.symdebug);
     global.params.targetTriple = llvm::Triple(gTargetMachine->getTargetTriple());
 
 #if LDC_LLVM_VER >= 302