Don't emit .loc with negative column numbers

The Clang assembler does not accept these. They can arise when using
Location.none, for example in a PPX.

Author: aantron

Changes

@@ -273,6 +273,9 @@ Compilers:
 - GPR#132: Flambda: new intermediate language and "middle-end" optimizers
   (Pierre Chambart, Mark Shinwell, Leo White)
 
+- GPR#212, PR#7226, GPR#542: emit column position in gas assembly `.loc`
+  (Frédéric Bour, Anton Bachin)
+
 - GPR#207: Colors in compiler messages (warnings, errors)
   configure with -color {auto|always|never} or TERM=dumb
   (Simon Cruanes, review by Gabriel Scherer)

asmcomp/x86_gas.ml

@@ -284,7 +284,9 @@ let print_line b = function
         file_num (X86_proc.string_of_string_literal file_name)
   | Indirect_symbol s -> bprintf b "\t.indirect_symbol %s" s
   | Loc (file_num, line, col) ->
-      bprintf b "\t.loc\t%d\t%d\t%d" file_num line col
+      (* PR#7726: Location.none uses column -1, breaks LLVM assembler *)
+      if col >= 0 then bprintf b "\t.loc\t%d\t%d\t%d" file_num line col
+      else bprintf b "\t.loc\t%d\t%d" file_num line
   | Private_extern s -> bprintf b "\t.private_extern %s" s
   | Set (arg1, arg2) -> bprintf b "\t.set %s, %a" arg1 cst arg2
   | Size (s, c) -> bprintf b "\t.size %s,%a" s cst c