Reorganize shims by platform
Fixes https://github.com/rust-lang/miri/issues/1161. I am currently testing which shims belong to which platform. If you are reading this and know the answer for any of those please write me :P
Edit: this is a list of the shims I haven't been able to figure out to which platform they belong:
-~~`malloc`~~
-~~`calloc`~~
-~~`posix_memalign`~~
-~~`free`~~
-~~`realloc`~~
-~~`__rust_alloc`~~
-~~`__rust_alloc_zeroed`~~
-~~`__rust_dealloc`~~
-~~`__rust_realloc`~~
-~~`__rust_maybe_catch_panic`~~
-~~`syscall`~~
-~~`getrandom`~~
-~~`dlsym`~~
-~~`memcmp`~~
-~~`memrchr`~~
-~~`memchr`~~
-~~`strlen`~~
-~~`signal`~~
-~~`sigaction`~~
-~~`sigaltstack`~~
-~~`sysconf`~~
-~~`sched_getaffinity`~~
-~~`isatty`~~
-~~`pthread_key_create`~~
-~~`pthread_key_delete`~~
-~~`pthread_getspecific`~~
-~~`pthread_setspecific`~~
-~~`pthread_attr_init`~~
-~~`pthread_attr_destroy`~~
-~~`pthread_self`~~
-~~`pthread_attr_setstacksize`~~
-~~`pthread_attr_getstack`~~
-~~`pthread_create`~~
-~~`pthread_mutexattr_init`~~
-~~`pthread_mutexattr_settype`~~
-~~`pthread_mutex_init`~~
-~~`pthread_mutexattr_destroy`~~
-~~`pthread_mutex_lock`~~
-~~`pthread_mutex_unlock`~~
-~~`pthread_mutex_destroy`~~
-~~`pthread_rwlock_rdlock`~~
-~~`pthread_rwlock_unlock`~~
-~~`pthread_rwlock_wrlock`~~
-~~`pthread_rwlock_destroy`~~
-~~`pthread_condattr_init`~~
-~~`pthread_condattr_setclock`~~
-~~`pthread_cond_init`~~
-~~`pthread_condattr_destroy`~~
-~~`pthread_cond_destroy`~~
-~~`pthread_atfork`~~
-~~`posix_fadvise`~~
-~~`mmap`~~
-~~`mprotect`~~
Add F_DUPFD/F_DUPFD_CLOEXEC to fcntl shim
This adds support for `F_DUPFD` and `F_DUPFD_CLOEXEC` to the shim for `fcntl`. (The `FileHandler` does not track the `FD_CLOEXEC` flag for open files, so these commands are effectively the same.) These changes enable using `File::try_clone`.
I also changed the initial value of the `low` field in `FileHandler`, so that it matches the intent of the preceding comment. The `open` shim was pre-incrementing it when choosing new file descriptor numbers, so FD 3 was being skipped.
