Improve syntax highlighting for AArch64 assembly

[kbeyls]

The syntax highlighting for the AArch64 assembly examples in the book isn't great.
At the moment, the gnuassembler language is selected to highlight these examples.

It doesn't recognize/highlight the AArch64 instructions correctly.
It also gets confused and highlights some operands as comments, see screenshots below

The syntax highlighting is done using pandoc's syntax highlighting mechanism. That pandoc documentation states:

If you are not satisfied with the built-in highlighting, or you want to highlight a language that isn’t supported, you can use the --syntax-definition option to load a KDE-style XML syntax definition file. Before writing your own, have a look at KDE’s repository of syntax definitions.

It looks like there is a 32-bit Arm syntax highlighting file, but no AArch64 one in that KDE repo: https://github.com/KDE/syntax-highlighting/blob/master/data/syntax/asm-arm.xml.