sim-se: only implement getdentsFunc on supported hosts

sim-se: only implement getdentsFunc on supported hosts

The implementation of the getdents syscall relies on SYS_getdents, which
is not available on all archs, because the getdents syscall has been
superseded by getdents64, and does not exist on newer archs such as
aarch64.

This leads the build to break on aarch64 hosts with error:

error: 'SYS_getdents' was not declared in this scope

Change-Id: I8701fb5b61c0418b14a9463ef135a391a7f7a9ba
Reviewed-on: https://gem5-review.googlesource.com/c/14596
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Brandon Potter <Brandon.Potter@amd.com>