Nowadays, if we use "compile" on aarch64-linux, we'll get the following
error,

(gdb) compile code -- ;
aarch64-none-linux-gnu-gcc: error: unrecognized command line option '-m64'

because the default gcc_target_options returns "-m64" and
"-mcmodel=large", neither is useful to aarch64-linux.

gdb:

2018-01-17  Yao Qi  <yao.qi@linaro.org>

	* aarch64-linux-tdep.c (aarch64_linux_gcc_target_options): New
	function.
	(aarch64_linux_init_abi): Install it to gdbarch hook
	gcc_target_options.