riscv: vdso_cfi: Add clean rule for copied sources

riscv: vdso_cfi: Add clean rule for copied sources

When building VDSO with CFI support, source files are copied from the main
VDSO directory to the CFI build directory as part of the build process.
However, these copied source files were not removed during 'make clean',
leaving temporary files in the build directory.

Add the clean-files variable to ensure that these copied .c and .S files
are properly cleaned up. The notdir() function is used to strip the path
prefix, as clean-files expects relative file names without directory
components.

This ensures the build directory is left in a clean state after make clean.

Signed-off-by: Chen Pei <cp0613@linux.alibaba.com>
Link: https://patch.msgid.link/20260320021850.1877-2-cp0613@linux.alibaba.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>