riscv: Introduce is_compat_thread() into compat.h

riscv: Introduce is_compat_thread() into compat.h

task_user_regset_view() makes use of a function very similar to
is_compat_task(), but pointing to a any thread.

In arm64 asm/compat.h there is a function very similar to that:
is_compat_thread(struct thread_info *thread)

Copy this function to riscv asm/compat.h and make use of it into
task_user_regset_view().

Also, introduce a compile-time test for CONFIG_COMPAT and simplify the
function code by removing the #ifdef.

Signed-off-by: Leonardo Bras <leobras@redhat.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Andy Chiu <andy.chiu@sifive.com>
Link: https://lore.kernel.org/r/20240103160024.70305-6-leobras@redhat.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>