S390: Fix infcalls in s390-vregs test case

S390: Fix infcalls in s390-vregs test case

GDB used to assume that functions without debug info return int.  It
accepted an expression containing such a function call and silently
interpreted the function's return value as int.  But nowadays GDB yields
an error message instead, see

  https://sourceware.org/ml/gdb-patches/2017-07/msg00139.html

This affects the s390-vregs test case, because it contains calls to
setrlimit64 and chdir.  When no glibc debug info is installed, these lead
to unnecessary FAILs.  Fix this by adding appropriate casts to the
inferior function calls.

gdb/testsuite/ChangeLog:

	* gdb.arch/s390-vregs.exp: Explicitly cast the return values of
	setrlimit and chdir to int.