arch-riscv: respect IALIGN, influenced by toggling 'c' extension.

arch-riscv: respect IALIGN, influenced by toggling 'c' extension.

According to the privileged ISA spec, SEPC[0]/MEPC[0] reads always 0
and SEPC[1]/MEPC[1] reads 0 if the compressed extension is disabled.

Additionally, the compressed extension can only be disabled if the next
instruction is 4-byte aligned.

Change-Id: I590c05e4000b59a5ba283f47933f7a92959d8e38
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25658
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>