arch: Define regular function calling convention register arguments

arch: Define regular function calling convention register arguments

Define the macros that one could use to access the registers within
`struct __regs` that represent the ABI/calling convention specific
that contain the arguments (and the return value).

Signed-off-by: Sergiu Moga <sergiu@unikraft.io>
Approved-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
GitHub-Closes: #1618