Add an arena backed by 1G pages for shared data not in low memory

Add an arena backed by 1G pages for shared data not in low memory

Summary:
Similar to the low 1G arena, but for data outside low memory.
malloc_huge() falls back to standard malloc() when space runs out.

I am not sure about the performance implications of bypassing thread cache.
Is it the only way to guarantee that allocation comes from the desired arena? cc jasone

Reviewed By: jasone

Differential Revision: D4180655

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