[gcc/fix-x32-mingw] 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

@@ -28,16 +28,6 @@
 
 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 hostPlatform targetPlatform;
@@ -57,6 +47,10 @@ let
       "--with-as=${if targetPackages.stdenv.cc.bintools.isLLVM then binutils else targetPackages.stdenv.cc.bintools}/bin/${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 [
       "--disable-libssp"
       "--disable-nls"
@@ -80,10 +74,6 @@ let
       "--disable-nls"
       # 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 [
       (if crossDarwin then "--with-sysroot=${lib.getLib libcCross}/share/sysroot"
        else                "--with-headers=${lib.getDev libcCross}${libcCross.incdir or "/include"}")