target/riscv: implement abstract command cache

Implemented cache of unsupported abstract commands.
It's purpose is to replace set of caching variables to one.
So this commit provides one ac_not_supported_cache
instead of abstract_read_csr_supported,
abstract_write_csr_supported, abstract_read_fpr_supported,
abstract_write_fpr_supported, has_aampostincrement.

Dropped check for buggy aampostincrement

Fixes #1232

Change-Id: I75cae1481841f2cd0393d6cc80f0d913fbe34294
Signed-off-by: Farid Khaydari <[email protected]>

Author: fk-sc