Auto merge of rust-lang#52281 - cramertj:fast-tls, r=alexcrichton

Use fast TLS on Fuchsia

I'm not sure why Fuchsia was separated here, but we provide these symbols, and tests are passing in QEMU with this change. cc @raphlinus.

r? @alexcrichton

src/libstd/sys/unix/fast_thread_local.rs

@@ -20,7 +20,7 @@
 // fallback implementation to use as well.
 //
 // Due to rust-lang/rust#18804, make sure this is not generic!
-#[cfg(target_os = "linux")]
+#[cfg(any(target_os = "linux", target_os = "fuchsia"))]
 pub unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern fn(*mut u8)) {
     use libc;
     use mem;
@@ -55,11 +55,6 @@ pub unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern fn(*mut u8)) {
     _tlv_atexit(dtor, t);
 }
 
-// Just use the thread_local fallback implementation, at least until there's
-// a more direct implementation.
-#[cfg(target_os = "fuchsia")]
-pub use sys_common::thread_local::register_dtor_fallback as register_dtor;
-
 pub fn requires_move_before_drop() -> bool {
     // The macOS implementation of TLS apparently had an odd aspect to it
     // where the pointer we have may be overwritten while this destructor