S390: Use soft float in s390-tdbregs test case

S390: Use soft float in s390-tdbregs test case

The GDB test case s390-tdbregs.exp verifies GDB's handling of the
"transaction diagnostic block".  For simplicity, the test case uses the
"transaction begin" (TBEGIN) instruction with the "allow floating-point
operation" flag set to zero.  But some GCC versions may indeed emit
floating point or vector instructions for this test case.  If this happens
in the transaction, it aborts, and an endless loop results.

This change tells the compiler to produce a soft-float binary, so no
floating-point or vector registers are touched.

gdb/testsuite/ChangeLog:

	* gdb.arch/s390-tdbregs.exp: Add the compile option -msoft-float.