Added mode for GNU assembler syntax.

The mode parses the GNU assembler syntax (also known as gas or AT&T
syntax.  An "architecture" option can be provided to the mode which
will cause the parser to accept machine specific syntax.  Currently
the only options accepted are "ARM", "ARMv6" and "x86".
[gas mode] Fix overly precise comparisons
[gas mode] Untabify
[gas mode] Integrate