Fix build breakage on GNU/Linux AArch64

Fix build breakage on GNU/Linux AArch64

This patch fixes the build breakage that has been happening on AArch64
since September 5th.  The breakage was introduced by the following
commit:

  author        Yao Qi <yao.qi@linaro.org>
          Tue, 5 Sep 2017 04:54:52 -0400 (09:54 +0100)
  committer     Yao Qi <yao.qi@linaro.org>
          Tue, 5 Sep 2017 04:54:52 -0400 (09:54 +0100)
  commit        f7000548a2b79d7e5cb924468117ca4245e6b820

  Use VEC for target_desc.reg_defs

The build log for this commit can be seen here:

  <https://gdb-build.sergiodj.net/builders/Ubuntu-AArch64-native-gdbserver-m64/builds/2696/steps/compile%20gdb/logs/stdio>

And the underlying problem is that the code is not calling the new
function "allocate_target_description" to allocate the "struct
target_desc" using "new" instead of XNEW, which end up not properly
initializing the fields of the structure.

Regtested on BuildBot.

gdb/gdbserver/ChangeLog:
2017-09-10  Sergio Durigan Junior  <sergiodj@redhat.com>

	* linux-low.c (handle_extended_wait): Use
	"allocate_target_description" instead of "XNEW".
	* linux-x86-low.c (initialize_low_arch): Likewise.