base/asm-common.h: Add some macros for shifting entire NEON vectors.

The `vext' (A32 NEON) or `ext' (A64) instructions can be (ab)used for
shifting vectors left and right if you have a spare zero vector lying
around.  But using them is kind of confusing: left shifts, especially,
need a reversed shift quantity, and the shift is measured in bytes
rather than bits.

Add a couple of macros to make this less strange.