std: add Darwin and FreeBSD sem_t bits

lib/std/c/darwin.zig

@@ -177,6 +177,7 @@ pub const pthread_cond_t = extern struct {
     __sig: c_long = 0x3CB0B1BB,
     __opaque: [__PTHREAD_COND_SIZE__]u8 = [_]u8{0} ** __PTHREAD_COND_SIZE__,
 };
+pub const sem_t = c_int;
 const __PTHREAD_MUTEX_SIZE__ = if (@sizeOf(usize) == 8) 56 else 40;
 const __PTHREAD_COND_SIZE__ = if (@sizeOf(usize) == 8) 40 else 24;
 

lib/std/c/freebsd.zig

@@ -47,6 +47,15 @@ pub const pthread_attr_t = extern struct {
     __align: c_long,
 };
 
+pub const sem_t = extern struct {
+    _magic: u32,
+    _kern: extern struct {
+        _count: u32,
+        _flags: u32,
+    },
+    _padding: u32,
+};
+
 pub const EAI = extern enum(c_int) {
     /// address family for hostname not supported
     ADDRFAMILY = 1,