incorrect usage of mmap return value in the standard library

```
../std/heap.zig:                if (addr == p.MAP_FAILED) return error.OutOfMemory;
../std/os.zig:    if (mmap_addr == posix.MAP_FAILED) return error.OutOfMemory;
```

These assume the libc API of mmap. However that is a libc thing. mmap returns an error in the normal syscall return code format and posix.getErrno() is needed to find out the error code.

Related: #2380 