arch,sim: Merge Process::syscall and Process::getDesc.

arch,sim: Merge Process::syscall and Process::getDesc.

When handling a system call, external code would call Process::syscall
which would extract the syscall number, that would call the base
class' doSyscall method, that would call into the subclass' getDesc
to get the appropriate descriptor, and then doSyscall would check
that a syscall was found and call into it.

Instead, we can just make the SyscallDescTable optionally check for
missing syscalls (in case we want to check multiple tables), and
make syscall look up the appropriate descriptor and call it. The base
implementation of syscall would then do the only bit of doSyscall that
is no longer being handled, incrementing the numSyscalls stat.

Change-Id: If102c156830ed2997d177dc6937cc85dddadf3f9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24119
Tested-by: kokoro <noreply+kokoro@google.com>
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Maintainer: Gabe Black <gabeblack@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>