[AIX] Emit error for -G option on AIX

1. Emit error for -G driver option on AIX
2. Adjust cmake file to use -Wl,-G instead of -G

On AIX, legacy XL compiler uses -G to produce a shared object enabled
for use with the run-time linker, which has different meanings from what
it is used for in Clang. And in Clang, other targets do not have -G map
to another functionality in their legacy compiler. So this error is more
important when we are on AIX.

Differential Revision: https://reviews.llvm.org/D89897