sim: Change the syscall executor to a std::function.

sim: Change the syscall executor to a std::function.

This will enable using other types of callable like a lambda.

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

Change-Id: If9f7176205492830824b5fe3c00f2c7710f57f70
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23164
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>