arch: Simplify the SyscallDesc tables.

arch: Simplify the SyscallDesc tables.

By using braced initializer lists and dropping the default
unimplementedFunc implementation function, the SyscallDesc tables
become a lot less crowded, and it's now very obvious which syscalls
are implemented just by quickly visually scanning the table.

This will also make it a lot easier to change the underlying type
stored in the table without having to adjust all of the instances
within them.

Jira Issue: https://gem5.atlassian.net/browse/GEM5-187

Change-Id: I7821de74812e1c02ca4550fc9c46cc2188cf1bd0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23189
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>