sepc register implementation issue

[fly-1011]

Bug Description:

While running NutShell, I discovered an issue with the handling of the lower bits of the sepc register.

According to the RISC-V specification:

The low bit of sepc (sepc[0]) is always zero. On implementations that support only IALIGN=32, the two low bits (sepc[1:0]) are always zero.

However, while running NutShell, the sepc value was 0xD (binary: 1101), which does not comply with the RISC-V specification.

Screenshots

Environment:

Nutshell version = commit 5dafeda
difftest version = commit 4ee1078f03c25ffe317d654a16cc814c7573151d