samples/ftrace: Fix asm function ELF annotations

Enable objtool coverage for the sample ftrace modules by adding ELF
annotations to the asm trampoline functions.

  samples/ftrace/ftrace-direct.o: warning: objtool: .text+0x0: unreachable instruction
  samples/ftrace/ftrace-direct-modify.o: warning: objtool: .text+0x0: unreachable instruction
  samples/ftrace/ftrace-direct-too.o: warning: objtool: .text+0x0: unreachable instruction

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>