arch-arm: Semihosting, specify files root dir

arch-arm: Semihosting, specify files root dir

This patch adds an option to "ArmSemihosting" which allows for
specifying an optional search path for host files.
Previously, behaviour was fixed to search in the directory from where
the gem5 binary was run from.

Change-Id: I57b932b38d022f132af78857104633d7bfdd1442
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23903
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>