arch-arm: Add support for semihosting STDIO redirection

arch-arm: Add support for semihosting STDIO redirection

The Arm Semihosting layer currently assumes that the guest application
shares STDIO with gem5. This makes it hard to distinguish application
output from gem5's output and makes it impossible to redirect STDIN
when running in interactive mode. Add support for custom STDIO
redirection when instantiating the Semihosting model.

Change-Id: I3411a6b9bfb008ffc3087d8837f59be72bd1e8ae
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Anouk Van Laer <anouk.vanlaer@arm.com>
Reviewed-by: Jack Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10021
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>