[gdbserver] Correctly generate i386-avx512.c

The makefile rule i386-avx512.c is to generate i386-avx512.c, but it
is written to i386-avx.c by mistake.  This patch is to fix this typo.

gdb/gdbserver:

2014-04-28  Yao Qi  <yao@codesourcery.com>

	* Makefile.in (i386-avx512.c): Fix the typo of generated file
	name.