tools build: Avoid circular .fixdep-in.o.cmd issues

The 'fixdep' tool is used to post-process dependency files for various
reasons, and it runs after every object file generation command. This
even includes 'fixdep' itself.

In Kbuild, this isn't actually a problem, because it uses a single
command to generate fixdep (a compile-and-link command on fixdep.c), and
afterward runs the fixdep command on the accompanying .fixdep.cmd file.

In tools/ builds (which notably is maintained separately from Kbuild),
fixdep is generated in several phases:

 1. fixdep.c -> fixdep-in.o
 2. fixdep-in.o -> fixdep

Thus, fixdep is not available in the post-processing for step 1, and
instead, we generate .cmd files that look like:

  ## from tools/objtool/libsubcmd/.fixdep.o.cmd
  # cannot find fixdep (/path/to/linux/tools/objtool/libsubcmd//fixdep)
  [...]

These invalid .cmd files are benign in some respects, but cause problems
in others (such as the linked reports).

Because the tools/ build system is rather complicated in its own right
(and pointedly different than Kbuild), I choose to simply open-code the
rule for building fixdep, and avoid the recursive-make indirection that
produces the problem in the first place.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Ian Rogers <irogers@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Link: https://lore.kernel.org/all/Zk-C5Eg84yt6_nml@google.com/
Link: https://lore.kernel.org/r/20240715203325.3832977-3-briannorris@chromium.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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