s390: Fix infcalls passing a single-field struct with static members

s390: Fix infcalls passing a single-field struct with static members

The infcall-nested-structs test case yields 36 FAILs on s390x because GCC
and GDB disagree on how to pass a C++ struct like this as an argument to a
function:

  struct s { float x; static float y; };

For the purpose of argument passing, GCC ignores static fields, while GDB
does not.  Thus GCC passes the argument in a floating-point register and
GDB passes it via memory.

Fix this by explicitly ignoring static fields when detecting single-field
structs.

gdb/ChangeLog:

	* s390-tdep.c (s390_effective_inner_type): Ignore static fields
	when unwrapping single-field structs.