Add intrinsics for float arithmetic with fast flag enabled

`fast` a.k.a UnsafeAlgebra is the flag for enabling all "unsafe"
(according to llvm) float optimizations.

See LangRef for more information http://llvm.org/docs/LangRef.html#fast-math-flags

Providing these operations with less precise associativity rules (for
example) is useful to numerical applications.

For example, the summation loop:

    let sum = 0.;
    for element in data {
        sum += *element;
    }

Using the default floating point semantics, this loop expresses the
floats must be added in a sequence, one after another. This constraint
is usually completely unintended, and it means that no autovectorization
is possible.

src/librustc_llvm/lib.rs

@@ -1310,6 +1310,7 @@ extern {
                          -> ValueRef;
     pub fn LLVMBuildNot(B: BuilderRef, V: ValueRef, Name: *const c_char)
                         -> ValueRef;
+    pub fn LLVMRustSetHasUnsafeAlgebra(Instr: ValueRef);
 
     /* Memory */
     pub fn LLVMBuildAlloca(B: BuilderRef, Ty: TypeRef, Name: *const c_char)