Option -mwarn_areg_zero for S/390.

[Doug Evans]

Martin Schwidefsky writes:
 > I recently added a warning to the S/390 assembler that is printed whenever
 > a specified base or index register is zero. That helped us to find some
 > critical bugs in the compiler. The background is that the general purpose
 > register 0 can't be used to address memory. I got several complaints about
 > these warnings and to make everybody happy I want to add a run-time option.
 > The default is not to print these messages and -mwarn_areg_zero enables
 > them.
 > 
 > ChangeLog for gas:
 > 
 > 2001-09-19  Martin Schwidefsky <schwidefsky@de.ibm.com>
 > 
 >      * gas/config/tc-s390.c: Add option -mwarn_areg_zero.

Why not -mwarn-areg-zero?

Having to remember which options are -mfoo-bar-baz
and which are -mfoo_bar_baz isn't something a user should have to do.