gcc: Fix MinGW exception handling so Rust works

reasoning:
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

(cherry picked from commit e27e475)
(cherry picked from commit 58ffaee)

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

@@ -24,6 +24,16 @@
 assert cloog != null -> stdenv.lib.versionOlder version "5";
 assert langJava -> stdenv.lib.versionOlder version "7";
 
+# 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
@@ -55,8 +65,11 @@ let
       "--with-gnu-as"
       "--with-gnu-ld"
       "--disable-debug"
-      "--enable-sjlj-exceptions"
       "--disable-win32-registry"
+    ] ++ 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"}")
@@ -78,11 +91,9 @@ let
       # musl at least, disable: https://git.buildroot.net/buildroot/commit/?id=873d4019f7fb00f6a80592224236b3ba7d657865
       "--disable-libmpx"
     ] ++ lib.optionals crossMingw [
-      "--enable-sjlj-exceptions"
       "--enable-hash-synchronization"
       "--enable-libssp"
       "--disable-nls"
-      "--with-dwarf2"
       # To keep ABI compatibility with upstream mingw-w64
       "--enable-fully-dynamic-string"
     ] ++ lib.optional (targetPlatform.libc == "newlib") "--with-newlib"