test_asm: added BNE.W
asm: added Label.off_* methods for usage in lambdas
asm: Label.offset: don't support bits=None

Because its behaviour is undefined for negative offsets
asm: Label.off_s: check for maximum value
asm: fixed Bxx.W, now works
asm: Label: new check methods for offset
asm: fixed CBx
test_asm: added test for CBZ, and new mock labels
test_asm: added test for CBNZ
test_asm: added test for B.W next