auto merge of #20980 : richo/rust/final-power, r=alexcrichton

Originally, this was going to be discussed and revisted, however I've been working on this for months, and a rebase on top of master was about 1 flight's worth of work so I just went ahead and did it.

This gets you as far as being able to target powerpc with, eg:

    LD_LIBRARY_PATH=./x86_64-unknown-linux-gnu/stage2/lib/ x86_64-unknown-linux-gnu/stage2/bin/rustc -C linker=powerpc-linux-gnu-gcc --target powerpc-unknown-linux-gnu hello.rs

Would really love to get this out before 1.0. r? @alexcrichton

src/libstd/os.rs

@@ -1422,6 +1422,11 @@ mod arch_consts {
     pub const ARCH: &'static str = "mipsel";
 }
 
+#[cfg(target_arch = "powerpc")]
+mod arch_consts {
+    pub const ARCH: &'static str = "powerpc";
+}
+
 #[cfg(test)]
 mod tests {
     use prelude::v1::*;