fix memory corruption when using standalone heap (#1478)

## Summary

Fix using the standalone heap (enabled with `--os:standalone` or
`-d:StandaloneHeapSize`) resulting in memory corruption due to
incorrect page alignment.

## Details

The initial pointer for the array-based page allocator is now aligned
to the size of a page, which ensures that `osAllocPages` always returns
a page-aligned pointer (the default allocator relies on this to be the
case).

A test is added to ensure that simple allocation/deallocation works
when using the array-based page allocator.

lib/system/osalloc.nim

@@ -183,7 +183,9 @@ elif hostOS == "standalone" or defined(StandaloneHeapSize):
   const StandaloneHeapSize {.intdefine.}: int = 1024 * PageSize
   var
     theHeap: array[StandaloneHeapSize div sizeof(float64), float64] # 'float64' for alignment
-    bumpPointer = cast[int](addr theHeap)
+    # pages must always be aligned to the page size, which the heap array is
+    # not guaranteed to be, hence the roundup
+    bumpPointer = roundup(cast[int](addr theHeap), PageSize)
 
   proc osAllocPages(size: int): pointer {.inline.} =
     if size+bumpPointer < cast[int](addr theHeap) + sizeof(theHeap):

tests/system/tosalloc_standalone_heap.nim

@@ -0,0 +1,14 @@
+discard """
+  description: "Ensure that the standalone heap option works"
+  matrix: "-d:StandaloneHeapSize:4166656 -d:useSysAssert"
+  targets: "c"
+"""
+
+# test with ``-d:useSysAssert`` so that violated allocator invariants are
+# caught
+
+# allocate, write, read back, and deallocate again
+let p = create(int)
+p[] = 100
+doAssert p[] == 100
+dealloc(p)