x86/tools: Use tools headers for instruction decoder selftests

Currently the x86 instruction decoder is used from:
- the kernel itself,
- from tools like objtool and perf,
- within x86 tools, i.e. instruction decoder selftests.

The first two cases are similar, because tools headers try to mimic
kernel headers.

Instruction decoder selftests include some of the kernel headers
directly, including uapi headers. This works until headers dependencies
are kept to a minimum and tools are not cross-compiled. Since the goal
of the x86 instruction decoder selftests is not to verify uapi headers,
move it to using tools headers, like is already done for vdso2c tool,
mkpiggy and other tools in arch/x86/boot/.

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>