kbuild: dummy-tools: pretend we understand -fpatchable-function-entry

Commit 0f71dcfb ("powerpc/ftrace: Add support for
-fpatchable-function-entry") added a script to check for
-fpatchable-function-entry compiler support. The script expects compiler
to emit the section __patchable_function_entries and few nops after a
function entry.

If the compiler understands and emits the above,
CONFIG_ARCH_USING_PATCHABLE_FUNCTION_ENTRY is set.

So teach dummy-tools' gcc about this.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>