Segmentation fault with threaded FFTW `FFTW.set_num_threads(n)` and `julia --threads=k` where `k, n > 1`

Hey,

I noticed that threaded FFTW.jl (1.0, 1.1, 1.3, 1.4) with Julia (1.6.1, 1.5.4) seems to crash with segmentation faults on my machine (AMD Ryzen 5 5600x). I have no clue what's going on, but it definitely worked in the past (a few weeks/months ago).

The following reproduces it on my system:

```julia
julia> FFTW.version # is that the linked library? How to change that?
v"3.3.9"

julia> versioninfo()
Julia Version 1.6.1
Commit 6aaedecc44 (2021-04-23 05:59 UTC)
Platform Info:
OS: Linux (x86_64-pc-linux-gnu)
CPU: AMD Ryzen 5 5600X 6-Core Processor
WORD_SIZE: 64
LIBM: libopenlibm
LLVM: libLLVM-11.0.1 (ORCJIT, generic)
Environment:
JULIA_NUM_THREADS = 12
JULIA_COPY_STACKS = yes
```

```julia
➜ julia --threads=1
using Revise, OhMyREPL
_
_ _ _(_)_ | Documentation: https://docs.julialang.org
(_) | (_) (_) |
_ _ _| |_ __ _ | Type "?" for help, "]?" for Pkg help.
| | | | | | |/ _` | |
| | |_| | | | (_| | | Version 1.6.1 (2021-04-23)
_/ |\__'_|_|_|\__'_| | Official https://julialang.org/ release
|__/ |

julia> begin
using FFTW
FFTW.set_num_threads(2)
x = randn((400, 400, 13))
fft(x);
end;

julia> begin
using FFTW
FFTW.set_num_threads(2)
x = randn((400, 400, 13))
fft(x);
end;

julia> begin
using FFTW
FFTW.set_num_threads(2)
x = randn((400, 400, 13))
fft(x);
end;

julia> begin
using FFTW
FFTW.set_num_threads(2)
x = randn((400, 400, 13))
fft(x);
end;

julia>
➜ julia --threads=2
using Revise, OhMyREPL
_
_ _ _(_)_ | Documentation: https://docs.julialang.org
(_) | (_) (_) |
_ _ _| |_ __ _ | Type "?" for help, "]?" for Pkg help.
| | | | | | |/ _` | |
| | |_| | | | (_| | | Version 1.6.1 (2021-04-23)
_/ |\__'_|_|_|\__'_| | Official https://julialang.org/ release
|__/ |

julia> begin
using FFTW
FFTW.set_num_threads(2)
x = randn((400, 400, 13))
fft(x);
end;

signal (11): Segmentation fault
in expression starting at REPL[1]:1

signal (11): Segmentation fault
in expression starting at REPL[1]:1
spawn_apply at /home/fxw/.julia/artifacts/81791030d1dcd08bf0c67b3e8224cb573d0f5a0a/lib/libfftw3.so (unknown line)
#2 at ./threadingconstructs.jl:169
unknown function (ip: 0x7fec7c04dabf)
_jl_invoke at /buildworker/worker/package_linux64/build/src/gf.c:2237 [inlined]
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2419
spawn_apply at /home/fxw/.julia/artifacts/81791030d1dcd08bf0c67b3e8224cb573d0f5a0a/lib/libfftw3.so (unknown line)
#2 at ./threadingconstructs.jl:169
unknown function (ip: 0x7fec7c04dabf)
_jl_invoke at /buildworker/worker/package_linux64/build/src/gf.c:2237 [inlined]
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2419
jl_apply at /buildworker/worker/package_linux64/build/src/julia.h:1703 [inlined]
start_task at /buildworker/worker/package_linux64/build/src/task.c:839
jl_init_root_task at /buildworker/worker/package_linux64/build/src/task.c:1288
jl_apply at /buildworker/worker/package_linux64/build/src/julia.h:1703 [inlined]
start_task at /buildworker/worker/package_linux64/build/src/task.c:839
_julia_init at /buildworker/worker/package_linux64/build/src/init.c:800
repl_entrypoint at /buildworker/worker/package_linux64/build/src/jlapi.c:702
main at julia (unknown line)
jl_init_root_task at /buildworker/worker/package_linux64/build/src/task.c:1288
jl_threadfun at /buildworker/worker/package_linux64/build/src/partr.c:264
start_thread at /lib/x86_64-linux-gnu/libpthread.so.0 (unknown line)
clone at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
Allocations: 5344071 (Pool: 5342120; Big: 1951); GC: 8
[1] 74566 segmentation fault (core dumped) julia --threads=2
```

Any idea what could go wrong? I mean it seems not to be very related to the julia version and FFTW.jl version.

Thanks,

Felix

**Edit**: On my laptop (Intel i7)with same versions, it seems to work fine