Implement ppc/ppc64 preserves_flags option for inline asm

Implemented preserves_flags on powerpc by making it do
nothing. This prevents having two different ways to mark
`cr0` as clobbered. clang and gcc alias `cr0` to `cc`.

The gcc inline documentation does not state what this does
on powerpc* targets, but inspection of the source shows
it is equivalent to condition register field `cr0`, so it
should not be added.

Author: pmur

src/asm.rs

@@ -546,9 +546,16 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
         }
 
         if !options.contains(InlineAsmOptions::PRESERVES_FLAGS) {
-            // TODO(@Commeownist): I'm not 100% sure this one clobber is sufficient
-            // on all architectures. For instance, what about FP stack?
-            extended_asm.add_clobber("cc");
+            match asm_arch {
+                InlineAsmArch::PowerPC | InlineAsmArch::PowerPC64 => {
+                    // "cc" is cr0 on powerpc.
+                }
+                // TODO(@Commeownist): I'm not 100% sure this one clobber is sufficient
+                // on all architectures. For instance, what about FP stack?
+                _ => {
+                    extended_asm.add_clobber("cc");
+                }
+            }
         }
         if !options.contains(InlineAsmOptions::NOMEM) {
             extended_asm.add_clobber("memory");