Add rules for socket_type enum values

[lucic71]

This adds support for translating socket_types values: SOCK_STREAM, SOCK_DGRAM, SOCK_CLOEXEC (Linux specific), SOCK_NONBLOCK (Linux specific).

The rules for CLOEXEC and NONBLOCK are only visible on Linux using the new IR field called target_os. On the C++ side, the rules are guarded with a macro:

#ifdef __linux__
int f4() {
  return SOCK_CLOEXEC;
}

int f5() {
  return SOCK_NONBLOCK;
}
#endif

On the Rust side, they are guarded using the target_os attribute:

#[cfg(target_os = "linux")]
unsafe fn f4() -> i32 {
    libc::SOCK_CLOEXEC
}

#[cfg(target_os = "linux")]
unsafe fn f5() -> i32 {
    libc::SOCK_NONBLOCK
}

rule-preprocessor parses the attributes of a function and adds target_os = linux in the IR. translation_rule.cpp skips the rule if the current OS is not the same as the OS declared by target_os.