x86: Instruction decoder test should generate build warning

Since some instructions are not decoded correctly by older
versions of objdump, it may cause false positive error in insn
decoder posttest.

This changes build error of insn decoder test to build warning.

Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
Cc: systemtap <systemtap@sources.redhat.com>
Cc: DLE <dle-develop@lists.sourceforge.net>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Randy Dunlap <rdunlap@xenotime.net>
Cc: Jim Keniston <jkenisto@us.ibm.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
LKML-Reference: <20091116230631.5250.41579.stgit@harusame>
Signed-off-by: Ingo Molnar <mingo@elte.hu>