Unbreak GDBserver build for x32

Unbreak GDBserver build for x32

When I verify my target description changes, I build GDB and GDBserver for
x32, but it failed.

/../../binutils-gdb/gdb/gdbserver/linux-amd64-ipa.c
../../../binutils-gdb/gdb/gdbserver/linux-amd64-ipa.c: In function ‘const target_desc* get_ipa_tdesc(int)’:
../../../binutils-gdb/gdb/gdbserver/linux-amd64-ipa.c:184:10: error: ‘X86_TDESC_AVX512’ was not declared in this scope
     case X86_TDESC_AVX512:
          ^
../../../binutils-gdb/gdb/gdbserver/linux-amd64-ipa.c:185:14: error: ‘tdesc_x32_avx512_linux’ was not declared in this scope
       return tdesc_x32_avx512_linux;
              ^
../../../binutils-gdb/gdb/gdbserver/linux-amd64-ipa.c: In function ‘void initialize_low_tracepoint()’:
../../../binutils-gdb/gdb/gdbserver/linux-amd64-ipa.c:282:36: error: ‘init_registers_x32_avx512_linux’ was not declared in this scope
   init_registers_x32_avx512_linux ();
                                    ^

ipa_x32_linux_regobj use to be there, but removed by
22049425ce40324139be82d9a6ec518c46b65815 by mistake.

gdb/gdbserver:

2017-08-04  Yao Qi  <yao.qi@linaro.org>

	* configure.srv (ipa_x32_linux_regobj): New.
	* linux-amd64-ipa.c (get_ipa_tdesc): Use X86_TDESC_AVX_AVX512
	instead of X86_TDESC_AVX512.
	(initialize_low_tracepoint): Call
	init_registers_x32_avx_avx512_linux.