Fix compiler check for stack unwind hint

The check inserts a DWARF directive to tell stack unwinding that the
bottom of the (co-routine) stack has been reached. Without this, stack
traces may attempt to continue past the bottom of the stack.

The GCC version check was incorrect, and failed to trigger for GCC
version 5.[0123].

mysys/my_context.c

@@ -206,7 +206,7 @@ my_context_spawn(struct my_context *c, void (*f)(void *), void *d)
     (
      "movq %%rsp, (%[save])\n\t"
      "movq %[stack], %%rsp\n\t"
-#if __GNUC__ >= 4 && __GNUC_MINOR__ >= 4 && !defined(__INTEL_COMPILER)
+#if (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 4)) && !defined(__INTEL_COMPILER)
      /*
        This emits a DWARF DW_CFA_undefined directive to make the return address
        undefined. This indicates that this is the top of the stack frame, and
@@ -456,7 +456,7 @@ my_context_spawn(struct my_context *c, void (*f)(void *), void *d)
     (
      "movl %%esp, (%[save])\n\t"
      "movl %[stack], %%esp\n\t"
-#if __GNUC__ >= 4 && __GNUC_MINOR__ >= 4 && !defined(__INTEL_COMPILER)
+#if (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 4)) && !defined(__INTEL_COMPILER)
      /*
        This emits a DWARF DW_CFA_undefined directive to make the return address
        undefined. This indicates that this is the top of the stack frame, and