split out pthread types for threads header

src/header/bits_pthreadonce-t/cbindgen.toml

@@ -0,0 +1,16 @@
+# POSIX header spec: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/sys_types.h.html
+#
+# This type is split out to prevent importing all of sys/types.h into other headers.
+#
+# POSIX headers that require pthread_once_t:
+# - pthread.h
+# - sys/types.h (where it should be defined)
+# - threads.h (not in spec but POSIX admits most implementations share definitions with pthread.h)
+include_guard = "_RELIBC_BITS_PTHREAD_ONCE_T_H"
+language = "C"
+no_includes = true
+cpp_compat = true
+[export]
+include = ["pthread_once_t"]
+[enum]
+prefix_with_name = true

src/header/bits_pthreadonce-t/mod.rs

@@ -0,0 +1,17 @@
+//! `pthread_once_t` for `sys/types.h` implementation.
+//!
+//! See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/sys_types.h.html>.
+
+use crate::{
+    platform::types::{c_int, c_uchar},
+    pthread_assert_equal_size,
+};
+
+/// Used for dynamic package initialization.
+#[repr(C)]
+pub union pthread_once_t {
+    __relibc_internal_size: [c_uchar; 4],
+    __relibc_internal_align: c_int,
+}
+
+pthread_assert_equal_size!(pthread_once_t, RlctOnce);