Issue #1 was caused because code_name and data_name had been incorrectly
defined as const-qualified pointers, but this must be an exception for
the z80 backend, as stated by the comments.
Issue #2 has been solved by removing the --gas switch from SDCCmain.c,
renaming it to "--asm=" (as requested by PKK) and moved to STM8's main.c,
so the option is limited to those targets supporting it.
Issue #4 was because of SDCCglue.c, that generated local labels using
"!labeldef" instead of "!slabeldef", causing those __str_%d labels to be
printed as "__str_%d::" (the double colons stand for globa label according
to asxxx documentation) instead of "__str_%d":".
I ran the regression tests and most of them passed, but "wchar" returned another
"Multiple definition of..." error that I cannot explain so far. It is better than
nothing, though.