Changing mblock size for wasm target increases failing tests

Currently, if the default MBLOCK_SHIFT is changed for wasm target, there are roughly 100 new test failures. This is not ideal and should be fixed some time in the future, for the following reasons:

• On any platform, the MBLOCK_SHIFT literal ought to be the single source of truth, and changing it should only affect performance of compiled code, not correctness.
• The default mblock size may not be ideal for wasm32. A smaller one may make more sense, e.g. changing it to page-sized (65536) will make it possible to drop a wasi-libc patch; also making it more practical to run the wasm in a rather resource-constrained runtime.

Below is one place that violates the single source of truth principle for MBLOCK_SHIFT, and there's likely more out there that ought to be fixed.

--- a/rts/sm/HeapAlloc.h
+++ b/rts/sm/HeapAlloc.h
@@ -64,7 +64,7 @@ extern struct mblock_address_range mblock_address_space;
 extern StgWord8 mblock_map[];
 
 /* On a 32-bit machine a 4KB table is always sufficient */
-# define MBLOCK_MAP_SIZE        4096
+# define MBLOCK_MAP_SIZE        (1 << (32 - MBLOCK_SHIFT))
 # define MBLOCK_MAP_ENTRY(p)    ((StgWord)(p) >> MBLOCK_SHIFT)
 # define HEAP_ALLOCED(p)        mblock_map[MBLOCK_MAP_ENTRY(p)]
 # define HEAP_ALLOCED_GC(p)     HEAP_ALLOCED(p)