gcc10: fix cross-compilation from aarch64-darwin host
gcc10: fix cross-compilation from aarch64-darwin host

Without this patch, linking when cross-compiling fails:

    Undefined symbols for architecture arm64:
      "_host_hooks", referenced from:
          gt_pch_save(__sFILE*) in libbackend.a(ggc-common.o)
          gt_pch_restore(__sFILE*) in libbackend.a(ggc-common.o)
          toplev::main(int, char**) in libbackend.a(toplev.o)
    ld: symbol(s) not found for architecture arm64

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