Fix incorrect maxStkSize calculation (#8858)

Signed-off-by: Austin Seipp <austin@well-typed.com>
(cherry picked from commit b99ace39)

rts/RtsFlags.c

@@ -97,12 +97,12 @@ void initRtsFlagsDefaults(void)
     StgWord64 maxStkSize = 8 * getPhysicalMemorySize() / 10;
     // if getPhysicalMemorySize fails just move along with an 8MB limit
     if (maxStkSize == 0)
-        maxStkSize = (8 * 1024 * 1024) / sizeof(W_);
+        maxStkSize = 8 * 1024 * 1024;
 
     RtsFlags.GcFlags.statsFile          = NULL;
     RtsFlags.GcFlags.giveStats          = NO_GC_STATS;
 
-    RtsFlags.GcFlags.maxStkSize         = maxStkSize;
+    RtsFlags.GcFlags.maxStkSize         = maxStkSize / sizeof(W_);
     RtsFlags.GcFlags.initialStkSize     = 1024 / sizeof(W_);
     RtsFlags.GcFlags.stkChunkSize       = (32 * 1024) / sizeof(W_);
     RtsFlags.GcFlags.stkChunkBufferSize = (1 * 1024) / sizeof(W_);