sim: Fix up the selectFunc syscall to work with g++ 10.2.

sim: Fix up the selectFunc syscall to work with g++ 10.2.

This is no longer willing to implicitly cast between the locally defined
Linux::fd_set type and the system fd_set type. That's pretty reasonable
since those types are really independent of one another, and we
shouldn't be using them interchangeably in the first place. That's a
pre-existing condition though, and I just want to get the existing code
to compile for now.

Change-Id: I41d5f3695dfe5f0e406d074d31d13c6e3282df64
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33415
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>