Use std::vector on tdesc->reg_defs (gdbserver/tdesc.h)

Use std::vector on tdesc->reg_defs (gdbserver/tdesc.h)

This is a followup patch to the build breakage fix on AArch64.  While
doing the fix, I found it better to convert tdesc->reg_defs (on
gdbserver/tdesc.h) from using VEC to using std::vector.  This makes
the code easier to read and maintain, and also is one more step
towards the C++fication.

Regtested on BuildBot.

2017-09-16  Sergio Durigan Junior  <sergiodj@redhat.com>

	* regcache.c (get_thread_regcache): Update code to use "std::vector"
	instead of "VEC" for "target_desc.reg_defs".
	(regcache_cpy): Likewise.
	(registers_to_string): Likewise.
	(registers_from_string): Likewise.
	(find_regno): Likewise.
	(supply_regblock): Likewise.
	(regcache_raw_read_unsigned): Likewise.
	* tdesc.c (init_target_desc): Likewise.
	(tdesc_create_reg): Likewise.
	* tdesc.h: Remove declaration of "tdesc_reg_p".  Include <vector>.
	(struct target_desc) <reg_defs>: Convert to "std::vector".
	(target_desc): Do not initialize "reg_defs".
	(~target_desc): Update code to use "std::vector" instead of "VEC"
	for "target_desc.reg_defs".
	(operator==): Likewise.