Julia assumes perm alloc won't trigger GC

[qinsoon]

The function mk_symbol is marked as JL_NOTSAFEPOINT, and it calls perm_alloc inside. The function with JL_NOTSAFEPOINT may not have its local variables not pushed to the shadow stack.
https://github.com/mmtk/julia/blob/f5f8510c016b40cb82f7445338bf9ef084704174/src/symbol.c#L34

We use MMTk for perm alloc, which may trigger GC. This will cause a correctness issue for us.

[qinsoon]

We disable GC before jl_get_layout which calls mk_symbol. I am not sure if this is necessary, but it complies with Julia's assumption.