arm64: Make L1_BLOCKS_SUPPORTED dynamic

[andrew]

Summary

When FEAT_LPA2 is implemented and the TCR_EL1.DS field is set we can
create blocks at level 1 with a 16kB page size. Add a variable to
tell the kernel this is enabled.

This will be used by other parts of pmap.c with a 4k page kernel so
make it visible there.

Sponsored by: Arm Ltd

[markj]

sys/arm64/arm64/pmap.c

1693

Maybe blocks_supported would be a better name.

[kib]

sys/arm64/arm64/pmap.c

369

Should both vars have __read_mostly annotation?

[alc]

sys/arm64/arm64/pmap.c

1693

I don't think that the blocks_ prefix is necessary. The other counter under the same sysctl node is about blocks too. In general, any of these sysctl nodes not ending in c, e.g., pmap.l1, pmap.l2, are about block mappings.

[andrew]

Use __read_mostly