cptbox: allow statx by default

[quantum5]

It used to only be allowed for compilers, but there is no reason why
it should be compiler-only given that other stat syscalls are allowed
for everything.

This will fix the failure with Turing on latest glibc.

[codecov-commenter]

Codecov Report

Merging #978 (bcc8a59) into master (f453014) will increase coverage by 2.93%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #978      +/-   ##
==========================================
+ Coverage   80.80%   83.74%   +2.93%     
==========================================
  Files         138      138              
  Lines        4695     4695              
==========================================
+ Hits         3794     3932     +138     
+ Misses        901      763     -138     

Impacted Files	Coverage Δ
dmoj/cptbox/isolate.py	86.55% <ø> (+47.05%)	⬆️
dmoj/executors/compiled_executor.py	77.71% <ø> (+4.00%)	⬆️
dmoj/judge.py	52.83% <0.00%> (+1.25%)	⬆️
dmoj/executors/java_executor.py	85.35% <0.00%> (+2.02%)	⬆️
dmoj/cptbox/tracer.py	77.33% <0.00%> (+18.34%)	⬆️
dmoj/cptbox/handlers.py	100.00% <0.00%> (+25.00%)	⬆️
dmoj/control.py	100.00% <0.00%> (+64.70%)	⬆️

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