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: I9690d9d79e3d1f593b63740b989074dcf0285637
Signed-off-by: Farid Khaydari <f.khaydari@syntacore.com>

Author: fkhaidari