gcc: use DWARF exception handling for i686-w64-mingw32

This is the default used by MSYS2[^1], ArchLinux[^2] and probably everyone else.
This is required to compile Rust programs as it doesn't support SJLJ[^3].

[^1]: https://github.com/msys2/MINGW-packages/blob/725f262e9f6313e3ee04327a33c16614c970a110/mingw-w64-gcc/PKGBUILD#L185-L186
[^2]: https://gitlab.archlinux.org/archlinux/packaging/packages/mingw-w64-gcc/-/blob/14.2.0-3/PKGBUILD#L62
[^3]: rust-lang/rust#55444 (comment)

Author: js6pak

pkgs/development/compilers/gcc/common/configure-flags.nix

@@ -36,15 +36,6 @@
 
 assert !enablePlugin -> disableGdbPlugin;
 
-# Note [Windows Exception Handling]
-# sjlj (short jump long jump) exception handling makes no sense on x86_64,
-# it's forcably slowing programs down as it produces a constant overhead.
-# On x86_64 we have SEH (Structured Exception Handling) and we should use
-# that. On i686, we do not have SEH, and have to use sjlj with dwarf2.
-# Hence it's now conditional on x86_32 (i686 is 32bit).
-#
-# ref: https://stackoverflow.com/questions/15670169/what-is-difference-between-sjlj-vs-dwarf-vs-seh
-
 let
   inherit (stdenv)
     buildPlatform
@@ -71,6 +62,10 @@ let
       }/bin/${targetPlatform.config}-as"
       "--with-ld=${targetPackages.stdenv.cc.bintools}/bin/${targetPlatform.config}-ld"
     ]
+    ++ lib.optionals (crossMingw && targetPlatform.isx86_32) [
+      "--disable-sjlj-exceptions"
+      "--with-dwarf2"
+    ]
     ++ (
       if withoutTargetLibc then
         [
@@ -98,11 +93,6 @@ let
           # To keep ABI compatibility with upstream mingw-w64
           "--enable-fully-dynamic-string"
         ]
-        ++ lib.optionals (crossMingw && targetPlatform.isx86_32) [
-          # See Note [Windows Exception Handling]
-          "--enable-sjlj-exceptions"
-          "--with-dwarf2"
-        ]
       else
         [
           (