[AIX] asm output: use character literals in byte lists for strings

[AIX] asm output: use character literals in byte lists for strings

This patch improves the assembly output produced for string literals by
using character literals in byte lists. This provides the benefits of
having printable characters appear as such in the assembly output and of
having strings kept as logical units on the same line.

Reviewed By: daltenty

Differential Revision: https://reviews.llvm.org/D80953