cc-wrapper: don't set -fno-omit-frame-pointer on s390/x390x

Without the change both `s390` and `s390x` `glibc` builds fail as
https://hydra.nixos.org/build/303208799:

In file included from ../sysdeps/s390/utf16-utf32-z9.c:501,
from ../sysdeps/s390/multiarch/utf16-utf32-z9.c:20:
../iconv/loop.c: In function '__from_utf16_loop_vx_cu':
../iconv/loop.c:274:1: error: %r11 cannot be used in 'asm' here
274 | }
| ^

Disable the `-fno-omit-frame-pointer` for now.

pkgs/build-support/cc-wrapper/default.nix

···
     # Do not prevent omission of framepointers on x86 32bit due to the small
     # number of general purpose registers. Keeping EBP available provides
     # non-trivial performance benefits.
+    # Also skip s390/s390x as it fails to build glibc and causes
+    # performance regressions:
+    #   https://bugs.launchpad.net/ubuntu-z-systems/+bug/2064538
+    #   https://github.com/NixOS/nixpkgs/issues/428260
     + (
       let
-        enable_fp = !targetPlatform.isx86_32;
+        enable_fp = !targetPlatform.isx86_32 && !targetPlatform.isS390;
         enable_leaf_fp =
           enable_fp
           && (