This patch implements global address lowering for 32/64 bit with small/large code model.
1.For 32bit large code model on AIX, there are newly added pseudo opcode LWZtocL & ADDIStocHA32, the support of which on MC layer will be provided by future patches.
2.The default code model on AIX should be small code model
3.Since AIX does not have medium code model, "report_fatal_error" when users specify it.
minor nit: we should probably check for SVR4 explicitly instead of if the object file format is ELF, just to be pedantic.