vm: centralize VM_BATCHQUEUE_SIZE definition

Remove the platform-specific definitions of VM_BATCHQUEUE_SIZE
for amd64 and powerpc64, and instead treat all 64-bit platforms
identically. This has the effect of increasing the arm64
and riscv VM_BATCHQUEUE_SIZE to match that of other platforms.

Reviewed by: jhb, markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D37707