Auto merge of rust-lang#104385 - BlackHoleFox:apple-minimum-bumps, r=petrochenkov

Raise minimum supported Apple OS versions

This implements the proposal to raise the minimum supported Apple OS versions as laid out in the now-completed MCP (rust-lang/compiler-team#556).

As of this PR, rustc and the stdlib now support these versions as the baseline:
- macOS: 10.12 Sierra
- iOS: 10
- tvOS: 10
- watchOS: 5 (Unchanged)

In addition to everything this breaks indirectly, these changes also erase the `armv7-apple-ios` target (currently tier 3) because the oldest supported iOS device now uses ARMv7s. Not sure what the policy around tier3 target removal is but shimming it is not an option due to the linker refusing.

[Per comment](rust-lang/compiler-team#556 (comment)), this requires a FCP to merge. cc `@wesleywiser.`

Author: bors

std/src/sys/unix/thread_local_dtor.rs

@@ -48,7 +48,7 @@ pub unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern "C" fn(*mut u8)) {
 // workaround below is to register, via _tlv_atexit, a custom DTOR list once per
 // thread. thread_local dtors are pushed to the DTOR list without calling
 // _tlv_atexit.
-#[cfg(target_os = "macos")]
+#[cfg(any(target_os = "macos", target_os = "ios", target_os = "watchos"))]
 pub unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern "C" fn(*mut u8)) {
     use crate::cell::Cell;
     use crate::mem;

std/src/sys/unix/thread_parking/darwin.rs

@@ -2,8 +2,7 @@
 //!
 //! Darwin actually has futex syscalls (`__ulock_wait`/`__ulock_wake`), but they
 //! cannot be used in `std` because they are non-public (their use will lead to
-//! rejection from the App Store) and because they are only available starting
-//! with macOS version 10.12, even though the minimum target version is 10.7.
+//! rejection from the App Store).
 //!
 //! Therefore, we need to look for other synchronization primitives. Luckily, Darwin
 //! supports semaphores, which allow us to implement the behaviour we need with