x86/arch_prctl: Simplify sys_arch_prctl()

Use in_ia32_syscall() instead of a compat syscall entry.

No change in functionality intended.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/20250202202323.422113-2-brgerst@gmail.com