PPC NCG: Reduce memory consumption emitting string literals

Emit string literals using the .string assembler directive and do proper escaping. Previously, we emitted a .byte directive for each character. This is worse in memory consumption than what X86 did before #14741 (closed) was fixed. As a nice side effect strings are now readable in the assembly file (well, assuming that most of us don't read decimal ASCII code :-).

All assemblers support .space for uninitialized memory and we simplify by removing the case statement.