[AIX] supporting the visibility attribute for aix assembly

SUMMARY:

in the aix assembly , it do not have .hidden and .protected directive.
in current llvm. if a function or a variable which has visibility attribute, it will generate something like the .hidden or .protected , it can not recognize by aix as.
in aix assembly, the visibility attribute are support in the pseudo-op like
.extern Name [ , Visibility ]
.globl Name [, Visibility ]
.weak Name [, Visibility ]

in this patch, we implement the visibility attribute for the global variable, function or extern function .

for example.

extern __attribute__ ((visibility ("hidden"))) int
  bar(int* ip);
__attribute__ ((visibility ("hidden"))) int b = 0;
__attribute__ ((visibility ("hidden"))) int
  foo(int* ip){
   return (*ip)++;
}
the visibility of .comm linkage do not support , we will have a separate patch for it.
we have the unsupported cases ("default" and "internal") , we will implement them in a a separate patch for it.

Reviewers: Jason Liu ,hubert.reinterpretcast,James Henderson

Differential Revision: https://reviews.llvm.org/D75866