riscv: delete temporary files

[ Upstream commit 95f4d9cced96afa9c69b3da8e79e96102c84fc60 ]

Temporary files used in the VDSO build process linger on even after make
mrproper: vdso-dummy.o.tmp, vdso.so.dbg.tmp.

Delete them once they're no longer needed.

Signed-off-by: Ilie Halip <ilie.halip@gmail.com>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>