riscv: s64ilp32: Add u32ilp32 ptrace support

The s64ilp32 supports both u64ilp32 and u32ilp32 ABIs, and their pt_regs
differ. So introduce the compat feature to help u32ilp32 ABI. Now
u64ilp32 and u32ilp32 applications could work with the s64ilp32 Linux
ptrace concurrently.

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