MIPS: Compat: Fix build error if CONFIG_MIPS32_COMPAT but no compat ABI.

In that case nor __NR_seccomp_*_32 symbols will be defined in
<asm/unistd.h> so the attempt to use it in kernel.seccomp.c will fail
with:

kernel/seccomp.c:565:2: error: '__NR_seccomp_read_32' undeclared here (not in a function)
  __NR_seccomp_read_32, __NR_seccomp_write_32, __NR_seccomp_exit_32, __NR_seccomp_sigreturn_32,
  ^
kernel/seccomp.c:565:24: error: '__NR_seccomp_write_32' undeclared here (not in a function)
  __NR_seccomp_read_32, __NR_seccomp_write_32, __NR_seccomp_exit_32, __NR_seccomp_sigreturn_32,
                        ^
kernel/seccomp.c:565:47: error: '__NR_seccomp_exit_32' undeclared here (not in a function)
  __NR_seccomp_read_32, __NR_seccomp_write_32, __NR_seccomp_exit_32, __NR_seccomp_sigreturn_32,
                                               ^
kernel/seccomp.c:565:69: error: '__NR_seccomp_sigreturn_32' undeclared here (not in a function)
  __NR_seccomp_read_32, __NR_seccomp_write_32, __NR_seccomp_exit_32, __NR_seccomp_sigreturn_32,

Solved by changing the compat ABIs in kconfig to select MIPS32_COMPAT
directly.  This also means the user no longer has to select MIPS32_COMPAT
before being able to see the ABI options.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>