[PowerPC64] restrict memcpy/bcopy optimization to POWER ISA >=V2.07

VSX instructions were added in POWER ISA V2.06 (POWER7), but it
requires data to be word-aligned. Such requirement was removed in
ISA V2.07B (POWER8).

Since current memcpy/bcopy optimization relies on VSX instructions
handling misalignment transparently, and kernel doesn't currently
implement an alignment error handler, this optimzation should be
restrict to ISA V2.07 onwards.

SIGBUS on stxvd2x instruction was reproduced in POWER7+ CPU.

Reviewed by: luporl, jhibbits, bdragon
Approved by: jhibbits (mentor)
Differential Revision: https://reviews.freebsd.org/D23958