Add Rust support to the syscall stub generator

In order to directly call syscalls from Rust, we need to expose the
assembly stubs as Rust extern functions. Add support for generating the
Rust declarations of these functions for use in the trusty-sys crate.

Test: build.py
Bug: 192403523
Change-Id: Ib26879b02647bc8b33cb6362bf7ebf83775e6c9f
1 file changed