fix powerpc build

Evidently, the powerpc gcc compiler feels it necessary to #define powerpc to 1. Seriously?  SERIOUSLY????

src/avian/common.h

@@ -27,6 +27,14 @@
 
 #include "float.h"
 
+#ifdef powerpc
+#  undef powerpc
+#endif
+
+#ifdef linux
+#  undef linux
+#endif
+
 // don't complain about using 'this' in member initializers:
 #  pragma warning(disable:4355)
 

src/codegen/target/powerpc/context.h

@@ -14,6 +14,9 @@
 #include <avian/vm/codegen/assembler.h>
 #include "avian/alloc-vector.h"
 
+#ifdef powerpc
+#undef powerpc
+#endif
 
 namespace vm {
 class System;

src/codegen/target/powerpc/encode.h

@@ -11,6 +11,10 @@
 #ifndef AVIAN_CODEGEN_ASSEMBLER_POWERPC_ENCODE_H
 #define AVIAN_CODEGEN_ASSEMBLER_POWERPC_ENCODE_H
 
+#ifdef powerpc
+#undef powerpc
+#endif
+
 namespace avian {
 namespace codegen {
 namespace powerpc {