COFF: use hash for string table also when copying / stripping

COFF: use hash for string table also when copying / stripping

Otherwise the string table may grow and hence e.g. change a final binary
(observed with PE/COFF ones) even if really there's no change. Doing so
in fact reduces the overall amount of code, and in particular the number
of places which need to remain in sync.

Afaics there's no real equivalent to the "traditional_format" field used
when linking, so hashing is always enabled when copying / stripping.