mips: Convert MIPS specific syscalls to Guest ABI.
mips: Convert MIPS specific syscalls to Guest ABI.

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

Change-Id: I7e7c49e885a8c3395f2e6ca361c228bce3691dbe
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23205
Reviewed-by: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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