Added 'gnuasm' lexer

[mpartel]

Adds a lexer for GNU Assembler.

This is based on nasm.rb and might retain some NASMisms. The most important differences to NASM: GNU Assembler directives start with ., registers start with %, and comment syntax is different.

This declares the same file extension '*.s' as 'armasm', ~so this PR removes armasm_spec.rb, which tested guessing by file extension. Is there a better way to handle this ambiguity?~ so added a disambiguation rule.

[mpartel]

Added disambiguation rule.