Re: r273910 build failure if <sys/_umtx.h> is out-of-date

I was just curious and decided to try an i386 build:
# make -C /usr/src buildworld TARGET=i386 -DNO_CCACHE
Gives same error:

/asp/git/src/lib/libc/../../include/semaphore.h:41:16: error: field has 
incomplete type 'struct _usem2'
         struct _usem2   _kern;      ^
/asp/git/src/lib/libc/../../include/semaphore.h:41:9: note: forward 
declaration
of 'struct _usem2'
         struct _usem2   _kern;   ^
cancelpoints_sem_new.c:66:33: error: use of undeclared identifier 
'USEM_MAX_COUNT'
_Static_assert(SEM_VALUE_MAX <= USEM_MAX_COUNT, "SEM_VALUE_MAX too 
large");    ^
cancelpoints_sem_new.c:335:15: warning: implicit declaration of function 
'USEM_COUNT' is invalid in C99 [-Wimplicit-function-declaration]
         *sval = (int)USEM_COUNT(sem->_kern._count);   ^
cancelpoints_sem_new.c:342:23: error: use of undeclared identifier 
'UMTX_OP_SEM2_WAKE'
         return _umtx_op(sem, UMTX_OP_SEM2_WAKE, 0, NULL, NULL);   ^
cancelpoints_sem_new.c:361:23: error: use of undeclared identifier 
'UMTX_OP_SEM2_WAIT'
         return _umtx_op(sem, UMTX_OP_SEM2_WAIT, 0,    ^
cancelpoints_sem_new.c:445:14: error: use of undeclared identifier 
'USEM_HAS_WAITERS'
         if (count & USEM_HAS_WAITERS)