powerpc: Add unwinder size REVISIT

Author: richo

src/libstd/rt/libunwind.rs

@@ -81,6 +81,9 @@ pub const unwinder_private_data_size: uint = 2;
 #[cfg(any(target_arch = "mips", target_arch = "mipsel"))]
 pub const unwinder_private_data_size: uint = 2;
 
+#[cfg(target_arch = "powerpc")]
+pub const unwinder_private_data_size: uint = 2;
+
 #[repr(C)]
 pub struct _Unwind_Exception {
     pub exception_class: _Unwind_Exception_Class,