The "ctags" code referencing tool creates an index for all identifiers
used in a project. The builtin recipes handle our C files just fine, but
due to a lack of a standard for marking functions and variables in assembly
files, will fail including the assembly code.

Provide the regular expressions that match the function tags used in our
assembly files, alongside the syntax we use for macros and "equ"
defines.

This will include assembly code in a ctags cross reference session.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: I47d531bfc6fafe9aeef9f4b66b7fdc4490b2e161