runtime: add missing persistentalloc memstats

[hzzb]

Before this commit, every persistentalloc call got one of memstats
statistics except when allocating mheap.arenas L2 array. If passing
nil sysStat into function persistentalloc, the memory allocated will
not be accounted.

Two changes after this commit:

1. Account mheap.arenas L2 array allocation into memestats.gcMiscSys
2. Fallback to memestats.other_sys if persistentalloc got nil sysStat

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Message from Michael Knyszek:

Patch Set 1:

(2 comments)

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Message from Michael Knyszek:

Patch Set 1:

(1 comment)

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Message from Michael Knyszek:

Patch Set 1:

(2 comments)

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Message from Michael Knyszek:

Patch Set 1:

(1 comment)

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After addressing review feedback, remember to publish your drafts!