arm64 pmap: Add ATTR_CONTIGUOUS support [Part 2]

Create ATTR_CONTIGUOUS mappings in pmap_enter_object(). As a result,
when the base page size is 4 KB, the read-only data and text sections
of large (2 MB+) executables, e.g., clang, can be mapped using 64 KB
pages. Similarly, when the base page size is 16 KB, the read-only
data section of large executables can be mapped using 2 MB pages.

Rename pmap_enter_2mpage(). Given that we have grown support for 16 KB
base pages, we should no longer include page sizes that may vary, e.g.,
2mpage, in pmap function names. Requested by: andrew

Co-authored-by: Eliot Solomon <ehs3@rice.edu>
Differential Revision: https://reviews.freebsd.org/D44575