Add entry_point_command struct to Mach-O definitions
Add entry_point_command struct to Mach-O definitions

The `entry_point_command` is a replacement for `thread_command`, and
is used for main executables to specify the location of `main()`
entry point.

Signed-off-by: Jakub Konka <6a6104c4f9eecd610ffe6bc75ff11e62c11e17d5@jakubkonka.com>

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