Remove all traces of __powerpc__

According to $GCC/gcc/config/rs6000/rs6000-c.c _ARCH_PPC is the
ubiquitous define which should be used to test whether gcc targets
PowerPC, on 64bit platforms _ARCH_PPC64 will be also defined.

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6301 c046a42c-6fe2-441c-8c8c-71466251a162
diff --git a/cpu-exec.c b/cpu-exec.c
index f574877..07ddd0e 100644
--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -1218,7 +1218,7 @@
                              &uc->uc_sigmask, puc);
 }
 
-#elif defined(__powerpc__)
+#elif defined(_ARCH_PPC)
 
 /***********************************************************************
  * signal context platform-specific definitions