Hack in acquire-load of task state to fix tests (#38557)

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)