trap is not raised on some unhandled misaligned memory accesses

[bieganski]

for example:

auipc x2, 1  # 0x80001000, naturally aligned
lw x1, 1(x2) # load 4 bytes from 0x80001001

will not raise and will return wrong result.

when issue is fixed remember about uncommenting relevant test in test_external_riscv.py.