highlight r8-r15b, append .fasm extension

* highlights `r8b-r15b`
* appends `*.fasm` extension

`r8b-r15b` are just the same as `r8l-r15l`

Some people use `.fasm` extension [[proof (it's download link)]](http://flatassembler.net/examples/fasm_amd64_linux64_samples.tar.gz)