[PowerPC] Remove unnecessary load of r12 in indirect call

When looking at the 64-bit SVR4 indirect call sequence, I noticed
an unnecessary load of r12.  And indeed the code says:

  // R12 must contain the address of an indirect callee.

But this is not correct; in the 64-bit SVR4 (ELFv1) ABI, there is
no need to load r12 at this point.  It seems this code and comment
is a remnant of code originally shared with the Darwin ABI ...

This patch simply removes the unnecessary load.