[mips64] Correct wasm address space limit.

MIPS64 has a user space of 2^40 bytes on most processors, address
space limits needs to be smaller. 256G is ok, but it will cause
mjsunit/wasm/bulk-memory-spec/memory_copy.wast get a OOM error.

Bug: v8:7747
Change-Id: Ie7a96336ef4ab6340def813478a09bdf3f76894e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1605524
Auto-Submit: Yu Yin <xwafish@gmail.com>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61417}

src/wasm/wasm-memory.cc

@@ -137,7 +137,7 @@ void* TryAllocateBackingStore(WasmMemoryTracker* memory_tracker, Heap* heap,
 #if V8_TARGET_ARCH_MIPS64
 // MIPS64 has a user space of 2^40 bytes on most processors,
 // address space limits needs to be smaller.
-constexpr size_t kAddressSpaceLimit = 0x4000000000L;  // 256 GiB
+constexpr size_t kAddressSpaceLimit = 0x8000000000L;  // 512 GiB
 #elif V8_TARGET_ARCH_64_BIT
 constexpr size_t kAddressSpaceLimit = 0x10100000000L;  // 1 TiB + 4 GiB
 #else