Fix stack buffer overflow in aarch64_extract_return_value

Fix stack buffer overflow in aarch64_extract_return_value

Hi,
I build GDB with -fsanitize=address, and run testsuite.  In
gdb.base/callfuncs.exp, I see the following error,

p/c fun1()
=================================================================^M
==9601==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7fffee858530 at pc 0x6df079 bp 0x7fffee8583a0 sp 0x7fffee858398
WRITE of size 16 at 0x7fffee858530 thread T0
    #0 0x6df078 in regcache_raw_read /home/yao/SourceCode/gnu/gdb/git/gdb/regcache.c:673
    #1 0x6dfe1e in regcache_cooked_read /home/yao/SourceCode/gnu/gdb/git/gdb/regcache.c:751
    #2 0x4696a3 in aarch64_extract_return_value /home/yao/SourceCode/gnu/gdb/git/gdb/aarch64-tdep.c:1708
    #3 0x46ae57 in aarch64_return_value /home/yao/SourceCode/gnu/gdb/git/gdb/aarch64-tdep.c:1918

We are extracting return value from V registers (128 bit), but only
allocate X_REGISTER_SIZE-byte array, which isn't sufficient.  This
patch changes the array to V_REGISTER_SIZE.

gdb:

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

	* aarch64-tdep.c (aarch64_extract_return_value):  Change array
	buf's length to V_REGISTER_SIZE.