perf bench, x86: Add alternatives-asm.h wrapper

perf bench needs this to build the kernel's memcpy routine:

In file included from bench/mem-memcpy-x86-64-asm.S:2:0:
bench/../../../arch/x86/lib/memcpy_64.S:7:33: fatal error: asm/alternative-asm.h: No such file or directory

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/n/tip-c5d41xibgullk8h2280q4gv0@git.kernel.orgSigned-off-by: NIngo Molnar <mingo@elte.hu>

tools/perf/util/include/asm/alternative-asm.h

+#ifndef _PERF_ASM_ALTERNATIVE_ASM_H
+#define _PERF_ASM_ALTERNATIVE_ASM_H
+
+/* Just disable it so we can build arch/x86/lib/memcpy_64.S for perf bench: */
+
+#define altinstruction_entry #
+
+#endif