arch-arm: Replace call to `tmpnam()` by a deterministic one

arch-arm: Replace call to `tmpnam()` by a deterministic one

According to the documentation, the use of tmpnam() should be
avoided.

This commit generates a temporary filename by concat-ing the
object name with an index that is internally tracked, the index
is increased until a filename that is not being used is found.

JIRA: https://gem5.atlassian.net/browse/GEM5-206

Change-Id: Ibfe604d741b6b7d7b02fc051add217f95f81d05e
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35195
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>