[GDBserver] Fix conversion warning

[GDBserver] Fix conversion warning

I got the following warning if I build GDBserver for aarch64_be-linux-gnu,

git/gdb/gdbserver/linux-aarch64-low.c:1539:39: error: invalid conversion from 'void*' to 'uint32_t* {aka unsigned int*}' [-fpermissive]
   uint32_t *le_buf = xmalloc (byte_len);
                                       ^
The patch is to fix the warning.

gdb/gdbserver:

2016-10-24  Yao Qi  <yao.qi@linaro.org>

	PR server/20733
	* linux-aarch64-low.c (append_insns): Cast the return value to
	'uint32_t *'.