CRASH in get_tls_thread_id in debug build on thread exit on AArch64

[derekbruening]

The app was run like this:

$ bin64/drrun -debug -no_enable_reset -disable_traces -vm_size 2G -msgbox_mask 0 -stderr_mask 15 -- <app and args>
...
Command terminated by signal 11

On this proprietary app I'm seeing this crash in 8 out of 10 runs in debug build (after fixing #3956). I do not see any crashes in release build.

Thread 23 "pthread-default" received signal SIGSEGV, Segmentation fault.
[Switching to LWP 169885]
0x0000fffff7e79f14 in get_tls_thread_id () at src/core/unix/os.c:2813
2813    src/core/unix/os.c: No such file or directory.
(gdb) bt
#0  0x0000fffff7e79f14 in get_tls_thread_id () at src/core/unix/os.c:2813
#1  0x0000fffff7e79eb0 in d_r_get_thread_id () at src/core/unix/os.c:2800
#2  0x0000fffff7c42688 in self_owns_recursive_lock (lock=0xfffff7f99bf0 <global_alloc_lock>) at src/core/utils.c:1062
#3  0x0000fffff7ccf95c in safe_to_allocate_or_free_heap_units () at src/core/heap.c:371
#4  0x0000fffff7cde15c in heap_free_unit (unit=0xffff6c3b0000, dcontext=0xffff6bc3cc00) at src/core/heap.c:3665
#5  0x0000fffff7cdf708 in threadunits_exit (tu=0xffff6bc3dc38, dcontext=0xffff6bc3cc00) at src/core/heap.c:3914
#6  0x0000fffff7cdfcb8 in heap_thread_exit (dcontext=0xffff6bc3cc00) at src/core/heap.c:4019
#7  0x0000fffff7bacc68 in dynamo_thread_exit_common (dcontext=0xffff6bc3cc00, id=169885, other_thread=false) at src/core/dynamo.c:2635
#8  0x0000fffff7bacddc in dynamo_thread_exit () at src/core/dynamo.c:2709
#9  0x0000fffff7e75bbc in cat_thread_only () at src/core/arch/aarch64/aarch64.asm:326

Xref #1931, now fixed, on ARM and WSL TLS issues?