Support solving for Dummy symbols in linsolve

[aktech]

The following works now:

In [4]: M = Matrix([[1, 2, 1, 1, 7], [1, 2, 2, -1, 12], [2, 4, 0, 6, 4]])

In [5]: system = A, b = M[:, :-1], M[:, -1]

In [8]: x1= Dummy('x1')

In [9]: x2= Dummy('x2')

In [10]: x3= Dummy('x3')

In [11]: x4= Dummy('x4')

In [12]: linsolve(system, x1, x2, x3, x4)
Out[12]: {(-2*_x2 - 3*_x4 + 2, _x2, 2*_x4 + 5, _x4)}

Fixes #9667
@hargup @flacjacket

[hargup]

+1