insert some newlines in generated code
don't multiply-define {send,recv}mmsg

These were already defined earlier in the file.
fix the generation of the syscall_defs table

The undefined-on-x86 syscalls shouldn't be included in this table; the
negative syscall numbers such syscalls have will cause out-of-bounds
accesses to other static tables, leading to segfaults.
avoid generating static_asserts for undefined-on-x86 syscalls