Hack in acquire-load of task state to fix tests

On x86 loads have implicit acquire semantics, but the same is
not true on other architectures (e.g. aarch64). We've been
observing test failures on aarch64 as a result (#38555).
Currently, we don't really have a good way to modify fields
in an atomic way (pending #37847), so this just achieves
the same thing using llvmcall to try and get the tests passing.

base/task.jl

@@ -131,10 +131,21 @@ const task_state_runnable = UInt8(0)
 const task_state_done     = UInt8(1)
 const task_state_failed   = UInt8(2)
 
+const _state_index = findfirst(==(:_state), fieldnames(Task))
+@eval function load_state_acquire(t)
+    # TODO: Replace this by proper atomic operations when available
+    @GC.preserve t llvmcall($("""
+        %ptr = inttoptr i$(Sys.WORD_SIZE) %0 to i8*
+        %rv = load atomic i8, i8* %ptr acquire, align 8
+        ret i8 %rv
+        """), UInt8, Tuple{Ptr{UInt8}},
+        Ptr{UInt8}(pointer_from_objref(t) + fieldoffset(Task, _state_index)))
+end
+
 @inline function getproperty(t::Task, field::Symbol)
     if field === :state
         # TODO: this field name should be deprecated in 2.0
-        st = getfield(t, :_state)
+        st = load_state_acquire(t)
         if st === task_state_runnable
             return :runnable
         elseif st === task_state_done
@@ -177,7 +188,7 @@ julia> istaskdone(b)
 true
 ```
 """
-istaskdone(t::Task) = t._state !== task_state_runnable
+istaskdone(t::Task) = load_state_acquire(t) !== task_state_runnable
 
 """
     istaskstarted(t::Task) -> Bool
@@ -221,7 +232,7 @@ true
 !!! compat "Julia 1.3"
     This function requires at least Julia 1.3.
 """
-istaskfailed(t::Task) = (t._state === task_state_failed)
+istaskfailed(t::Task) = (load_state_acquire(t) === task_state_failed)
 
 Threads.threadid(t::Task) = Int(ccall(:jl_get_task_tid, Int16, (Any,), t)+1)