[wasm][stdlib] Add wasi-libc support for Heap.cpp

Pointers returned by malloc() are 16-byte aligned on wasi-libc, even
it's 32-bit architecture. See wasi-libc's dlmalloc configuration:
https://github.com/WebAssembly/wasi-libc/blob/aecd368c6dedc417037afa136139eccc4490e56e/dlmalloc/src/dlmalloc.c#L31

Author: kateinoigakukun

stdlib/public/runtime/Heap.cpp

@@ -32,10 +32,11 @@ using namespace swift;
 /// On Apple platforms, \c malloc() is always 16-byte aligned.
 static constexpr size_t MALLOC_ALIGN_MASK = 15;
 
-#elif defined(__linux__) || defined(_WIN32)
+#elif defined(__linux__) || defined(_WIN32) || defined(__wasi__)
 /// On Linux and Windows, \c malloc() returns 16-byte aligned pointers on 64-bit
 /// and 8-byte aligned pointers on 32-bit.
-#if defined(__LP64) || defined(_WIN64)
+/// On wasi-libc, pointers are 16-byte aligned even though 32-bit for SIMD access.
+#if defined(__LP64) || defined(_WIN64) || defined(__wasi__)
 static constexpr size_t MALLOC_ALIGN_MASK = 15;
 #else
 static constexpr size_t MALLOC_ALIGN_MASK = 7;