Native debug arm program by aarch64 GDB

Native debug arm program by aarch64 GDB

This patch is to let aarch64 GDB debug 32-bit arm program natively.  In
each function for fetching and storing registers, GDB will check
gdbarch_bfd_arch_info (gdbarch)->bits_per_word, if it is 32, call
the corresponding aarch32 functions in aarch32-linux-nat.c, otherwise
fall back to aarch64 code to fetch and store registers.

aarch64_linux_read_description has to return the right target description,
but we don't have gdbarch available there, so GDB fetches auxv and gets
AT_PHENT, in order to determine whether the target is 32-bit or 64-bit.
I learned this trick from solib-svr4.c.

gdb:

2015-07-07  Yao Qi  <yao.qi@linaro.org>

	* aarch32-linux-nat.h (VFP_REGS_SIZE): New macro, moved from
	arm-linux-nat.c.
	* aarch64-linux-nat.c: Include aarch32-linux-nat.h and
	elf/external.h.
	(fetch_gregs_from_thread): Call aarch32_gp_regcache_supply
	if target is 32-bit.
	(store_gregs_to_thread): Call aarch32_gp_regcache_collect
	if target is 32-bit.
	(fetch_fpregs_from_thread): Call aarch32_vfp_regcache_supply
	if target is 32-bit.
	(store_fpregs_to_thread): Call aarch32_vfp_regcache_collect
	if target is 32-bit.
	(tdesc_arm_with_vfpv3, tdesc_arm_with_neon): Declare.
	(aarch64_linux_read_description): Return the right target
	description.
	* arm-linux-nat.c (VFP_REGS_SIZE): Moved to aarch32-linux-nat.h.
	* config/aarch64/linux.mh (NATDEPFILES): Add aarch32-linux-nat.o.
	* configure.tgt (aarch64*-*-linux*): Add arm-tdep.o and
	arm-linux-tdep.o