rts: Allocate MBlocks with MAP_TOP_DOWN on Windows

As noted in #18991, we would previously allocate heap in low memory.
Due to this the linker, which typically *needs* low memory, would end up
competing with the heap. In longer builds we end up running out of
low memory entirely, leading to linking failures.

(cherry picked from commit a1a75aa9)

rts/win32/OSMem.c

@@ -67,8 +67,11 @@ allocNew(uint32_t n) {
     alloc_rec* rec;
     rec = (alloc_rec*)stgMallocBytes(sizeof(alloc_rec),"getMBlocks: allocNew");
     rec->size = ((W_)n+1)*MBLOCK_SIZE;
+    // N.B. We use MEM_TOP_DOWN here to ensure that we leave the bottom of the
+    // address space available for the linker and libraries, which in general
+    // want to live in low memory. See #18991.
     rec->base =
-        VirtualAlloc(NULL, rec->size, MEM_RESERVE, PAGE_READWRITE);
+        VirtualAlloc(NULL, rec->size, MEM_RESERVE | MEM_TOP_DOWN, PAGE_READWRITE);
     if(rec->base==0) {
         stgFree((void*)rec);
         rec=0;