[Backend] Rewrite code for linking external library to expose more inlining opportunities (#1037)

- Also make it cleaner. 
- And mark out the code needs to be fixed in `semantic.py`.

python/triton/language/semantic.py

@@ -1116,11 +1116,13 @@ def xor_sum(input: tl.tensor, axis: int, builder: ir.builder) -> tl.tensor:
 
 def umulhi(x: tl.tensor, y: tl.tensor, builder: ir.builder) -> tl.tensor:
     x, y = binary_op_type_checking_impl(x, y, builder)
+    # FIXME(Keren): not portable, should be fixed
     from . import libdevice
     return libdevice.mulhi(x, y, _builder=builder)
 
 
 def floor(x: tl.tensor, builder: ir.builder) -> tl.tensor:
+    # FIXME(Keren): not portable, should be fixed
     from . import libdevice
     return libdevice.floor(x, _builder=builder)