[binutils-gdb] catch-syscall.exp: Fix missing architecture name for s390:31-bit

[sergiodj+buildbot at redhat dot com]

*** TEST RESULTS FOR COMMIT 6d74a497ba4fc2ba67142310a9fad42e035ac9a7 ***

Author: Andreas Arnez <arnez@linux.vnet.ibm.com>
Branch: master
Commit: 6d74a497ba4fc2ba67142310a9fad42e035ac9a7

catch-syscall.exp: Fix missing architecture name for s390:31-bit
For the "multiple targets" test in catch-syscall.exp, set the 'arch1'
variable to a valid string.

gdb/testsuite/ChangeLog:

	* gdb.base/catch-syscall.exp (test_catch_syscall_multi_arch): Set
	the 'arch1' variable for "s390*-linux*" targets.