Use VEC for target_desc.reg_defs

Nowadays, target_desc.reg_defs is a pointer points to a pre-generated
array, which is not flexible.  This patch changes it from an array
to a VEC so that GDBserver can create target descriptions dynamically
later.  Instead of using pre-generated array, the -generated.c calls
VEC_safe_push to add each register to vector.

Since target_desc.reg_defs is used in IPA, we need to build common/vec.c
for IPA too.

gdb/gdbserver:

2017-09-05  Yao Qi  <yao.qi@linaro.org>

	* Makefile.in (IPA_OBJS): Add vec-ipa.o
	* regcache.c (get_thread_regcache): Use VEC_length.
	(init_register_cache): Likewise.
	(regcache_cpy): Likewise.
	(registers_to_string): Iterate reg_defs via VEC_iterate.
	(find_regno): Likewise.
	(find_register_by_number): Use VEC_index.
	(register_size): Call find_register_by_number.
	(register_data): Call find_register_by_number.
	(supply_regblock): Use VEC_length.
	(regcache_raw_read_unsigned): Likewise.
	* tdesc.c (init_target_desc): Iterate reg_defs via
	VEC_iterate.
	(default_description): Update initializer.
	(copy_target_description): Don't update field num_registers.
	* tdesc.h (struct target_desc) <reg_defs>: Change it to VEC.
	<num_registers>: Remove.

gdb:

2017-09-05  Yao Qi  <yao.qi@linaro.org>

	* regformats/regdat.sh: Update generated code.