MIPS: asm: syscall: Define syscall_get_arch

This effectively renames __syscall_get_arch to syscall_get_arch
and implements a compatible interface for the seccomp API.
The seccomp code (kernel/seccomp.c) expects a syscall_get_arch
function to be defined for every architecture, so we drop
the leading underscores from the existing function.

This also makes use of the 'task' argument to determine the type
the process instead of assuming the process has the same
characteristics as the kernel it's running on.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Reviewed-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6398/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>