[patch] Add discriminator support to gas .loc directive

[Cary Coutant]

>> Tested on i386. Is this OK for trunk?
>
> Almost, but not quite:
>
>  * Please add a line to gas/NEWS mentioning the new feature and extend the
> description of the .loc directive in gas/doc/as.texinfo.

OK.

>  * Why do you allow the user to set a value of 0 for the descriminator when
> only values greater than 0 are emitted ?

The default value of the discriminator "register" in the line table
state machine is 0, and is reset to that with each row that is
emitted, so we don't need to set the register to 0 when it's already
0.

> PS.  When you submit the binutils/dwarf.c patch please could you also extend
> the gas/testsuite/gas/lns/lns-common-1.s test to cover the new sub-command.

Will do. I was looking for the right place to add a test for this.

-cary