scripts/checkstack.pl: match all stack sizes for s390

For some unknown reason the regular expression for checkstack only matches
three digit numbers starting with the number "3", or any higher
number. Which means that it skips any stack sizes smaller than 304
bytes. This makes the checkstack script a bit less useful than it could be.

Change the script to match any number. To be filtered out stack sizes
can be configured with the min_stack variable, which omits any stack
frame sizes smaller than 100 bytes by default.

Tested-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>