Separate out the command execution part of if_changed, as we did
for if_changed_dep.

This allows us to reuse it in if_changed_rule.

  define rule_foo
          $(call cmd_and_savecmd,foo)
          $(call cmd,bar)
  endef

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # LLVM-14 (x86-64)