objtool: Add WARN_INSN()

It's easier to use and also gives easy access to the instruction's
containing function, which is useful for printing that function's
symbol.  It will also be useful in the future for rate-limiting and
disassembly of warned functions.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/2eaa3155c90fba683d8723599f279c46025b75f3.1681325924.git.jpoimboe@kernel.org