microblaze: Use the correct select syscall (BZ #28883)

On Microblaze only __NR_newselect is implemented, even though kernel
advertise __NR_select on asm/unistd.h.  Since microblaze is the
only architecture that undef __ASSUME_PSELECT, the generic code
change is simpler than chaging the architecture syscall number.

Acked-by: Mark Hatle <mark.hatle@xilinx.com>