Determine target description for native aarch64

Determine target description for native aarch64

I find the following test fail when I test native aarch64 gdb with
arm program,

(gdb) PASS: gdb.base/attach-pie-noexec.exp: attach
set architecture arm^M
warning: Selected architecture arm is not compatible with reported target architecture aarch64^M
Architecture `arm' not recognized.^M
The target architecture is set automatically (currently aarch64)^M
(gdb) FAIL: gdb.base/attach-pie-noexec.exp: set architecture arm

GDB thinks the target is aarch64, but it isn't.  Nowadays, we are
using some entries AT_PHENT and AT_HWCAP in auxv to determine whether
the process is a 32-bit arm one or 64-bit aarch64 one, and get the
right gdbarch.  However, in the process of parsing auxv (in
inf_ptrace_auxv_parse), the size of int and data pointer of
target_gdbarch is used.  If debug program exists (in most of cases),
target_gdbarch is already set according to the debug program, which
is arm in my case.  Then, GDB can parse auxv successfully.  However,
in gdb.base/attach-pie-noexec.exp, the debug program is removed,
target_gdbarch is aarch64 when GDB parse auxv, so GDB can't parse
it successfully.

Instead of using auxv, we check the return value of ptrace NT_ARM_VFP.
If the program is an arm process, NT_ARM_VFP is OK, otherwise, error
is returned.

Additionally, we only return tdesc_arm_with_neon for arm process,
because neon is mandatory on ARMv8.

gdb:

2016-08-04  Yao Qi  <yao.qi@linaro.org>

	* aarch64-linux-nat.c (tdesc_arm_with_vfpv3): Remove the
	declaration.
	(aarch64_linux_read_description): Remove code on getting
	auxv and select target description on it.  Select target
	description by the result of NT_ARM_VFP ptrace request.