Use value_contents instead of value_contents_writeable

Use value_contents instead of value_contents_writeable

Both aarch64_push_dummy_call and bfin_push_dummy_call only use args[i]
contents but then never write to them, so that we can use
value_contents instead.

gdb:

2015-11-16  Yao Qi  <yao.qi@linaro.org>

	* aarch64-tdep.c (aarch64_push_dummy_call): Call value_contents instead
	of value_contents_writeable.
	* bfin-tdep.c (bfin_push_dummy_call): Likewise.