simh: add -fcommon workaround

Without the change -fno-common toolchains like upstream gcc-10
or clang-11+ fail to build simh as:

    $ nix build --impure --expr 'with import ~/nm {}; simh.override { stdenv = clang13Stdenv; }'
    ...
    ld: /build/pdp11_io-0c74f0.o:(.bss+0x10c00):
      multiple definition of `uc15_memsize'; /build/pdp11_cis-17339e.o:(.bss+0x118): first defined here
    ld: /build/pdp11_io_lib-296a06.o:(.bss+0x8000):
      multiple definition of `uc15_memsize'; /build/pdp11_cis-17339e.o:(.bss+0x118): first defined here

pkgs/applications/emulators/simh/default.nix

@@ -22,6 +22,11 @@ stdenv.mkDerivation rec {
 
   dontConfigure = true;
 
+  # Workaround to build against upstream gcc-10 and clang-11.
+  # Can be removed when next release contains
+  #    https://github.com/simh/simh/issues/794
+  NIX_CFLAGS_COMPILE = [ "-fcommon" ];
+
   makeFlags = [ "GCC=${stdenv.cc.targetPrefix}cc" "CC_STD=-std=c99" "LDFLAGS=-lm" ];
 
   preInstall = ''