gas: have scrubber also respect quoted labels

For the handling of ':' elsewhere in the scrubber to be correct with
regard to labels, the state after parsing a string found at the start of
a line must match that after finding a symbol character at the start of
a line. (Things are largely okay when there's whitespace ahead of the
label: Whitespace after the colon then is retained rather than dropped
for typical targets like x86, but read.c will know to deal with that.)