[AArch64] Split OP_prfm opcode, or provide named constants for the sub-opcode immediate operand

See the discussion at https://github.com/DynamoRIO/dynamorio/pull/4386#discussion_r462356954 regarding opcode philosophy of separate opcodes for separate semantics.

This issue covers either splitting the OP_prfm opcode into `OP_prfm_read_l1`, `OP_prfm_read_l1_nt`, etc., with a helper to determine whether an opcode is a `PRFM`, or leaving the single OP_prfm and providing a named enum where `DR_PRFM_READ_L1_NT`, etc. are set to equal their encoded 5-bit immediate value in the source operand. Both approaches avoid the bit decoding currently at https://github.com/DynamoRIO/dynamorio/blob/master/clients/drcachesim/tracer/instru.cpp#L82 and we'd want to update that code with the result. We have pros and cons of both approaches; splitting the opcodes is a compatibilty break as well. Whoever takes on the work gets to pick the approach I suppose.