Thread safety issue of pipe read&write

Please include the following in your bug report:

**Version of emscripten/emsdk:**
3.1.54

**Failing command line in full:**
```shell
# emscripten
emcc pipe.cpp -pthread -o main.js
node main.js

# g++
g++ pipe.cpp -o main
./main

```

Hi, I was using the `pipe` in a multi-threading scenario. I found if I write a pipe in one thread, then read in another thread, it seems the process will not work as expected.

In the following code, it uses a variable, `cnt` to count the number of `"xy"` read from the pipe. Here, `cnt` is expected to be `100` because in the write end, we write `"xy"` 100 times to the pipe, and so does it perform in the native version.

However, in the wasm execution, the result is not as expected as shown below. The value of `cnt` is not `100`, and it even can't get a stable result (found to be 24, 22, 0, etc.), which seems to indicate a possible thread safety issue.

It'd be great if you could check it. Thank you!

#### Code
```c++
#include
#include
#include
#include
#include
#include
#include
int main() {
int fds[2] = {};

if (pipe(fds) == -1) {
perror("pipe error");
}

std::thread write_thread([&]() {
std::string input = "xy";
for (int i = 0; i < 100; ++i) {
int len = write(fds[1], input.c_str(), input.size());
if(len <= 0) perror("write error");
}
close(fds[1]);
});

std::atomic cnt(0);

auto fun = [&]() {
char value[2];
int len = 0;
while ((len = read(fds[0], value, sizeof(value))) > 0) {
if(!(value[0] == 'x' && value[1] == 'y') || len != 2) {
break;
}
++cnt;
}
};
std::cout << "Is there error?: " << strerror(errno) << std::endl;

std::thread read_thread(fun);
write_thread.join();

read_thread.join();

std::cout << "cnt: " << cnt << std::endl;
close(fds[0]);
return 0;
}
```
#### Results
Here is the result of wasm and native execution. (For the wasm result, here it just shows a sample: here the result of wasm execution is actually not stable and could be various values.)
```shell
# emscripten
Is there error?: No error information
count: 22

# g++
Is there error?: Success
cnt: 100
```