S390: Improve comments for s390-tdbregs test case

S390: Improve comments for s390-tdbregs test case

This adds more explanation as to why the test case must be compiled with
the -msoft-float option.  It also documents the my_tbegin and my_tend
functions.

gdb/testsuite/ChangeLog:

	* gdb.arch/s390-tdbregs.c (my_tbegin): Add comment documenting the
	function.
	(my_tend): Likewise.
	* gdb.arch/s390-tdbregs.exp: Enhance comment; explain the
	rationale of avoiding FP- and vector instructions.