target/riscv: make mem_access_result_t enum type safe

Make mem_access_result_t enum type safe and fix related problems

Checkpatch-ignore: MACRO_ARG_PRECEDENCE, MULTISTATEMENT_MACRO_USE_DO_WHILE
Checkpatch-ignore: TRAILING_SEMICOLON
Change-Id: Ie5a8c71f3a8ad803f1660114c399c5a4dd0f7414
Signed-off-by: Farid Khaydari <f.khaydari@syntacore.com>

Author: fk-sc