riscv: u64ilp32: Add signal support for compat

The u64ilp32 reuses compat mode on the 64-bit Linux kernel, but the
signal context is the same as the native 64-bit, not u32ilp32. So use
the native signal procedure for u64ilp32 applications.

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