Machine: fix instruction to_str

[trdthg]

negative hex should use '-' in the str, or it will be treated as uint

jal x1, -0xFF:

str::asHex((int32_t)-0xff) -> s: 0xffffffffffffff01
str::asHex((uint32_t)-0xff) -> s: 0xffffff01

'-' lost after to_str()