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

[alfredo]

Summary

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, we should restrict this to ISA V2.07 onwards.

SIGBUS on stxvd2x instruction was reproduced in POWER7+ CPU.

Test Plan

Buildworld and chroot
Installworld and reboot

[bdragon]

lib/libc/powerpc64/string/bcopy_resolver.c

64–69

Perhaps put a comment here explaining why this was chosen, so nobody's tempted to change it back to PPC_FEATURE_HAS_VSX a couple years down the road when we've forgotten this.

[alfredo]

Added code comments

[luporl]

LGTM

[bdragon]

Looks fine to me as well.