Write output to stdout if GASPP_DEBUG env var is set
Fix thumb function handling

Correctly detect branch targets by matching all forms of branch
instructions and allowing whitespace around a label on the same
line.  Emit .thumb_func directives for known targets before the
first use, otherwise they are ignored.
Fix use of symbols in literals

The Apple linker does not support relocations against an offset
from a symbol.  Using an intermediate local symbol works around
this limitation.