riscv: u64ilp32: Enable user space runtime switch

This patch didn't introduce any new syscall table but reused the
existing rv32 implementation to ease the maintenance of the kernel side.
Unify the UXL mode setting by ELF e_flags to support u64ilp32 &
u32ilp32.

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