cosmopolitan: Update compiler and update platform_internal.h (#3079)

This fixes the cosmopolitan platform.
- Switch `build_cosmocc.sh` and platform documentation to
  explicitly use the x86_64 cosmocc compiler as multi-arch
  cosmocc won't work here. Older version `cosmocc` just did
  a x86_64 build.
- Add missing items from `platform_internal.h` to fix build.

core/shared/platform/cosmopolitan/platform_internal.h

@@ -55,6 +55,7 @@ typedef pthread_t korp_tid;
 typedef pthread_mutex_t korp_mutex;
 typedef pthread_cond_t korp_cond;
 typedef pthread_t korp_thread;
+typedef pthread_rwlock_t korp_rwlock;
 typedef sem_t korp_sem;
 
 #define OS_THREAD_MUTEX_INITIALIZER PTHREAD_MUTEX_INITIALIZER
@@ -67,6 +68,12 @@ typedef int os_file_handle;
 typedef DIR *os_dir_stream;
 typedef int os_raw_file_handle;
 
+static inline os_file_handle
+os_get_invalid_handle()
+{
+    return -1;
+}
+
 #if WASM_DISABLE_WRITE_GS_BASE == 0
 #if defined(BUILD_TARGET_X86_64) || defined(BUILD_TARGET_AMD_64)
 #define os_writegsbase(base_addr)                                 \