dyndb test broken on 9.16 when building without libtool

The change to lock the trampoline while attaching introduced in commit be7f672f has caused the dyndb system test to fail; ns1 crashes on shutdown.

#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#1  0x00007f974017a859 in __GI_abort () at abort.c:79
#2  0x0000561ece79d1e8 in assertion_failed (
    file=0x561ecebb0602 "trampoline.c", line=154, 
    type=isc_assertiontype_require, 
    cond=0x561ecebb06b8 "trampolines[trampoline->tid] == trampoline")
    at ./main.c:272
#3  0x0000561eceab0dde in isc_assertion_failed (
    file=0x561ecebb0602 "trampoline.c", line=154, 
    type=isc_assertiontype_require, 
    cond=0x561ecebb06b8 "trampolines[trampoline->tid] == trampoline")
    at assertions.c:48
#4  0x0000561eceafdd0a in isc__trampoline_detach (trampoline=0x561ecf5162f0)
    at trampoline.c:154
#5  0x0000561eceafdf16 in isc__trampoline_run (arg=0x561ecf5162f0)
    at trampoline.c:202
#6  0x00007f9740363609 in start_thread (arg=<optimized out>)
    at pthread_create.c:477
#7  0x00007f9740277163 in clone ()
    at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

trampolines[trampoline->tid] is set to NULL.