rust: fix rust cross-compile

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

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

@@ -58,8 +58,9 @@ let
       "--with-gnu-as"
       "--with-gnu-ld"
       "--disable-debug"
-      "--enable-sjlj-exceptions"
       "--disable-win32-registry"
+    ] ++ lib.optionals (crossMingw && targetPlatform.isx86_32) [
+      "--enable-sjlj-exceptions"
     ] else [
       (if crossDarwin then "--with-sysroot=${lib.getLib libcCross}/share/sysroot"
        else                "--with-headers=${lib.getDev libcCross}${libcCross.incdir or "/include"}")
@@ -81,13 +82,14 @@ 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.optionals (crossMingw && targetPlatform.isx86_32) [
+      "--enable-sjlj-exceptions"
+      "--with-dwarf2"
     ] ++ lib.optional (targetPlatform.libc == "newlib") "--with-newlib"
       ++ lib.optional (targetPlatform.libc == "avrlibc") "--with-avrlibc"
     );