riscv: u64ilp32: Adjust vDSO alternative for 64ilp32 abi

The 64ilp32 uses the same ELF32 as 32ilp32 and the 64lp64 uses ELF64, so
separate apply_vdso_alternatives into 64 and 32 versions and serve for
three kinds of vDSO - vdso32, vdso64, vdso64ilp32.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>