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

0 files changed, 0 insertions, 0 deletions