arch: Stop using setSyscallArg to set argc and argv.

arch: Stop using setSyscallArg to set argc and argv.

In Alpha and MIPS, the argc and argv values should be in what happens
to be the first and second syscall argument registers, but that's not
by definition. The process objects of both those ISAs know what
registers to use intrinsically, so there's also no reason to call out
to a helper method which acts as a part of the Process's interface to
the rest of gem5.

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

Change-Id: Id8fa38ab1fc2ac6436e94ad41303439973fded10
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23173
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>