mips64-linux: declare it an n32 ABI target

    Without the change build on mips64-unknown-linux-gnu fails as:

        $ nix-build -A buildPackages.gcc12 --argstr crossSystem mips64-linux

        In file included from ...-glibc-mips64-unknown-linux-gnu-2.37-8-dev/include/bits/stat.h:25,
                         from ...-glibc-mips64-unknown-linux-gnu-2.37-8-dev/include/fcntl.h:78,
                         from ../../../../gcc-12.3.0/libsanitizer/sanitizer_common/sanitizer_linux.cpp:55:
        ...-glibc-mips64-unknown-linux-gnu-2.37-8-dev/include/bits/struct_stat.h:190:8: error: redefinition of 'struct stat64'
          190 | struct stat64
              |        ^~~~~~

lib/systems/parse.nix

@@ -474,6 +474,8 @@ rec {
             else abis.gnueabi
           # Default ppc64 BE to ELFv2
           else if isPower64 parsed && isBigEndian parsed then abis.gnuabielfv2
+          # gcc default mips64-unknown-linux-gnu to n32 ABI, not 64-bit ABI.
+          else if isMips64 parsed then abis.gnuabin32
           else abis.gnu
         else                     abis.unknown;
     };