libc/aarch64: Add memset for a 64 byte dc zva

On arm64 we can use the "dc zva" instruction to zero memory. The CPU
tells software if the instruction is implemented, and if so the size
and alignment

libc/aarch64: Add memset for a 64 byte dc zva

On arm64 we can use the "dc zva" instruction to zero memory. The CPU
tells software if the instruction is implemented, and if so the size
and alignment it will use.

When the size is 64-bytes the Arm Optimized Routines implementation of
memset can use dc zva to zero memory, and has a build flag to skip
checking.

Use this flag to build a version of memset that will be used when this
assumption is true.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D54776

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