use a trampoline to get to the spicy _x functions

internal/msgx/msgx_darwin.s

@@ -0,0 +1,9 @@
+//go:build !ios
+
+#include "textflag.h"
+
+// The same trampoline shape x/sys/unix uses for its libSystem stubs; it assembles for amd64 and arm64 alike.
+TEXT msgx_dlsym_trampoline<>(SB),NOSPLIT,$0-0
+	JMP	msgx_dlsym(SB)
+GLOBL	·dlsymTrampolineAddr(SB), RODATA, $8
+DATA	·dlsymTrampolineAddr(SB)/8, $msgx_dlsym_trampoline<>(SB)