to test the following compiler CFLAGS

1. -fno-omit-frame-pointer without -mapcs
	will reserve fp, but not the whole "struct stackframe" like kernel

2. -fno-omit-frame-pointer -mapcs
	will reserve {fp, ip, lr, pc} like kernel, at the high end of
	current stack frame.
	[ip, fp] will be the scope of caller's stack frame;
	lr hold current stack frame's, i.e. current function's return address;
	pc seems useless;

3. -fomit-frame-pointer
	will not reserve fp