bpf: Revert ("Fix dispatcher patchable function entry to 5 bytes nop")

Because __attribute__((patchable_function_entry)) is only available
since GCC-8 this solution fails to build on the minimum required GCC
version.

Undo these changes so we might try again -- without cluttering up the
patches with too many changes.

This is an almost complete revert of:

  dbe69b29 ("bpf: Fix dispatcher patchable function entry to 5 bytes nop")
  ceea991a

 ("bpf: Move bpf_dispatcher function out of ftrace locations")

(notably the arch/x86/Kconfig hunk is kept).

Reported-by: David Laight <David.Laight@aculab.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Björn Töpel <bjorn@kernel.org>
Tested-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Björn Töpel <bjorn@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lkml.kernel.org/r/439d8dc735bb4858875377df67f1b29a@AcuMS.aculab.com
Link: https://lore.kernel.org/bpf/20221103120647.728830733@infradead.org