Fix __ASSUME_PSELECT for MicroBlaze (bug 16642).

Reviewing (for all architectures, with a baseline kernel version of
2.6.32) the kernel support for features for which __ASSUME_* macros
would be affected by a move to 2.6.32 as minimum kernel version showed
up that __ASSUME_PSELECT was wrongly defined for MicroBlaze, despite
the corresponding syscall table entry not being wired up in the
MicroBlaze kernel.

This patch makes the MicroBlaze kernel-features.h undefine
__ASSUME_PSELECT.  I'd also encourage wiring it up in the kernel (so
you can then make this #undef conditional, and eventually obsolete
once a recent-enough kernel is required).  I suspect it wasn't wired
up because of the mistaken comment in asm/unistd.h "obsolete ->
sys_pselect7" (there is no such syscall as pselect7).

	[BZ #16642]
	* sysdeps/unix/sysv/linux/microblaze/kernel-features.h
	(__ASSUME_PSELECT): Undefine.