Fix 13048 - Solaris no longer needs -mt during linking

As stated in the bug issue, -mt is no longer needed as of Solaris 10 (I don't think it'd be worth trying to target earlier versions given the added complexity of separate single/multi-threaded process models and how old they are now).

It also prevents the use of non-Oracle/Sun studio compilers as they don't recognize the -mt option.

https://issues.dlang.org/show_bug.cgi?id=13048