Mark codeinstance.specsigflags as atomic

[vchuravy]

A correctness fix from #53255

This shouldn't matter much as that field is
only accessed from the runtime, but this
synchronizes the C-header definition, with
the Julia native definition.

[inkydragon]

CI error related: :linux: test x86_64-linux-gnu

core                                             (11) |         failed at 2024-02-11T02:31:01.327
Test Failed at /cache/build/tester-amdci5-9/julialang/julia-master/julia-810f560bdf/share/julia/test/core.jl:46
  Expression: Set((fieldname(T, i) for i = 1:fieldcount(T) if Base.isfieldatomic(T, i))) == Set(c)
   Evaluated: Set([:purity_bits, :max_world, :specptr, :next, :inferred, :isspecsig, :min_world, :precompile, :invoke]) == Set([:purity_bits, :max_world, :specptr, :next, :inferred, :precompile, :min_world, :specsigflags, :invoke])