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/cl_options.h

@@ -16,6 +16,7 @@
 #define LDC_DRIVER_CL_OPTIONS_H
 
 #include "mars.h"
+#include "driver/target.h"
 #include "llvm/Support/CodeGen.h"
 #include "llvm/Support/CommandLine.h"
 #include <deque>
@@ -60,6 +61,7 @@ namespace opts {
     extern cl::opt<std::string> mTargetTriple;
     extern cl::opt<llvm::Reloc::Model> mRelocModel;
     extern cl::opt<llvm::CodeModel::Model> mCodeModel;
+    extern cl::opt<FloatABI::Type> mFloatABI;
     extern cl::opt<bool, true> singleObj;
     extern cl::opt<bool> linkonceTemplates;