sysvipc: Fix tst-sysvshm-linux on x32

The Linux shminfo fields are '__syscall_ulong_t' (which is 64-bit
for x32).  This patch fixes the test to compare againt the correct
type and to only clamp the value if '__syscall_ulong_t' is the same
size of 'unsigned long int'.

Checked on x86_64-linux-gnu-x32.