[PowerPC] Parse and ignore .machine ppc64

In the wild, kexec-tools purgatory/arch/ppc64/v2wrap.S and hvcall.S
use this directive.

Author: MaskRay

llvm/lib/Target/PowerPC/AsmParser/PPCAsmParser.cpp

@@ -1737,7 +1737,7 @@ bool PPCAsmParser::ParseDirectiveMachine(SMLoc L) {
   // Implement ".machine any" (by doing nothing) for the benefit
   // of existing assembler code.  Likewise, we can then implement
   // ".machine push" and ".machine pop" as no-op.
-  if (CPU != "any" && CPU != "push" && CPU != "pop")
+  if (CPU != "any" && CPU != "push" && CPU != "pop" && CPU != "ppc64")
     return TokError("unrecognized machine type");
 
   Parser.Lex();

llvm/test/MC/PowerPC/ppc-machine.s

@@ -14,3 +14,4 @@
 	.machine "any"
 	.machine "pop"
 
+	.machine ppc64