wasm: don't include stdint.h for __wasi__

because wasi-libc's `stdint.h` is highjacked by libcxx's `stdint.h`
tries to load the next `stdint.h` and it results in loading whole `std` C++
standard library. However, loading whole std during building Swift
stdlib through SwiftShims causes cyclic reference among Swift stdlib
types and libc.

Author: kateinoigakukun

stdlib/public/SwiftShims/swift/shims/SwiftStdint.h

@@ -24,7 +24,7 @@
 
 // Clang has been defining __INTxx_TYPE__ macros for a long time.
 // __UINTxx_TYPE__ are defined only since Clang 3.5.
-#if !defined(__APPLE__) && !defined(__linux__) && !defined(__OpenBSD__)
+#if !defined(__APPLE__) && !defined(__linux__) && !defined(__OpenBSD__) && !defined(__wasi__)
 #include <stdint.h>
 typedef int64_t __swift_int64_t;
 typedef uint64_t __swift_uint64_t;