runtime: initialize the memory limit in mallocinit

Currently the memory limit is left uninitialized before gcinit, and
allocations may happen. The result is that the span allocation path
might try to scavenge memory unnecessarily. Prevent this by setting the
memory limit up early to its default value.

Change-Id: I886d9a8fa645861e4f88e0d54af793418426f520
Reviewed-on: https://go-review.googlesource.com/c/go/+/450736
Run-TryBot: Michael Knyszek <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Michael Knyszek <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: mknyszek

src/runtime/malloc.go

@@ -571,6 +571,9 @@ func mallocinit() {
 		userArenaHint.addr = p
 		userArenaHint.next, mheap_.userArena.arenaHints = mheap_.userArena.arenaHints, userArenaHint
 	}
+	// Initialize the memory limit here because the allocator is going to look at it
+	// but we haven't called gcinit yet and we're definitely going to allocate memory before then.
+	gcController.memoryLimit.Store(maxInt64)
 }
 
 // sysAlloc allocates heap arena space for at least n bytes. The