Fix __ASSUME_PREADV and __ASSUME_PWRITEV for Alpha and MicroBlaze (bug 16649).

Fix __ASSUME_PREADV and __ASSUME_PWRITEV for Alpha and MicroBlaze (bug 16649).

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_PREADV and __ASSUME_PWRITEV were wrongly defined for
MicroBlaze (despite the corresponding syscall table entries not being
wired up in the kernel) and Alpha for 2.6.30 and above (although the
support on Alpha was added in 2.6.33).  This patch makes the
kernel-features.h files undefine those macros for appropriate
versions.

	[BZ #16649]
	* sysdeps/unix/sysv/linux/alpha/kernel-features.h
	[__LINUX_KERNEL_VERSION < 0x020621] (__ASSUME_PREADV): Undefine.
	[__LINUX_KERNEL_VERSION < 0x020621] (__ASSUME_PWRITEV): Likewise.
	* sysdeps/unix/sysv/linux/microblaze/kernel-features.h
	(__ASSUME_PREADV): Undefine.
	(__ASSUME_PWRITEV): Likewise.