Make .isa-file ##include file paths relative to including file.

Make .isa-file ##include file paths relative to including file.
Makes .isa files cleaner and simplifies scanner too.
Simplified scanner to work under both old and new versions of scons.

arch/SConscript:
    Simplify .isa scanner... seems to work with both scons 0.96.1 and 0.96.91 now.
    Assumes .isa ##include paths are relative to including file.
arch/alpha/isa/main.isa:
arch/mips/isa/formats/formats.isa:
arch/mips/isa/main.isa:
arch/sparc/isa/formats.isa:
arch/sparc/isa/main.isa:
    Make ##include paths relative to including file.
arch/isa_parser.py:
    Make ##include file paths relative to including file.
    Makes .isa files cleaner and simplifies scanner too.
    Partial rewrite of include-handling code to use cool re.sub() feature
    where you can specify a function to provide the replacement string.
    Minor cleanup of error-handling code.
    Also got rid of '#!' at top to make caller choose which python interpreter
    is used (since SPARC now requires 2.4 to build, we may need to do that via
    scons in the future).

--HG--
rename : arch/mips/isa/formats.isa => arch/mips/isa/formats/formats.isa
extra : convert_revision : 15a3920fa3aaf80cd94083eda853aa4e49425045