Switch macOS task mechanism back to ASM (JuliaLang#39679)

5327824 rearranged the ifdefs here and
switched macOS to unwind based switching, which is somewhat slower. That was
deemed acceptable at the time, because it work around some GDB misbehavior
in setjmp. However, we're about to update to the LLVM libunwind fork, which
appears to be even slower here, so switch back to ASM to avoid that performance
regression.

src/julia_threads.h

@@ -35,10 +35,12 @@ typedef win32_ucontext_t jl_ucontext_t;
      defined(_CPU_ARM_) || defined(_CPU_PPC64_))
 #define JL_HAVE_ASM
 #endif
-#if defined(_OS_DARWIN_)
-#define JL_HAVE_UNW_CONTEXT
+#if 0
+// very slow, but more debugging
+//#elif defined(_OS_DARWIN_)
+//#define JL_HAVE_UNW_CONTEXT
 //#elif defined(_OS_LINUX_)
-//#define JL_HAVE_UNW_CONTEXT // very slow, but more debugging
+//#define JL_HAVE_UNW_CONTEXT
 #elif defined(_OS_EMSCRIPTEN_)
 #define JL_HAVE_ASYNCIFY
 #elif !defined(JL_HAVE_ASM)