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