AArch64 opnd_create_abs_addr() succeeds but opnd_is_memory_reference() returns false

For #4134 and #3995 I'm porting drbbdup to arm and its tests call `opnd_create_abs_addr()` to create the `runtime_case_opnd`. DR lets it create an absolute addr opnd of type `ABS_ADDR_kind` on AArch64; but then `opnd_is_memory_reference` returns false on AArch64 because the check for `opnd_is_abs_addr()` is limited to `IF_X86_64`. Basically, DR isn't sure it wants to support absolute address operands for AArch64, since the addressing modes there do not have that sort of thing. Maybe users are expected to use `opnd_create_rel_addr()` instead (AArch64 does have `#define OPND_CREATE_ABSMEM(addr, size) opnd_create_rel_addr(addr, size)`): but the docs for `opnd_create_abs_addr()` say it will auto-convert into a rel addr.

The rest of the code for abs addr handling in opnd_shared.c is just gated by `X64`, so `opnd_is_memory_reference` looks like an anomaly. So maybe the solution is to fix `opnd_is_memory_reference` to return true, and ensure the AArch64 and ARM encoders handle `ABS_ADDR_kind` and treat it just like rel-addr (the alternative of converting to rel-addr at creation time might confuse users who would query their own just-created abs-addr opnd and it would return false for `opnd_is_abs_addr()`).

I went to check whether the AArch64 encoder might already handle abs-addr: but it looks like it doesn't even handle rel-addr! See
https://github.com/DynamoRIO/dynamorio/issues/4847#issuecomment-1023433975