syscall_emul: fix openat when directory does not end in "/"

syscall_emul: fix openat when directory does not end in "/"

Before this commit, the following code:

    dir_fd = open(".", O_DIRECTORY);
    file_fd = openat(dir_fd, "ble", O_CREAT, S_IRUSR | S_IWUSR);

would create a file called ".ble" in the current working directory,
instead of the correct "ble".

Change-Id: I1525a088d49744e29b760387afabef9f1ac98646
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/13005
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Brandon Potter <Brandon.Potter@amd.com>