Auto merge of #30987 - antonblanchard:powerpc64_merge3, r=alexcrichton

We currently pass generic as the CPU to LLVM. This results in worse
than required code generation. On little endian, which is only POWER8,
we avoid many POWER4 and newer instructions.

Pass ppc64 and ppc64le instead.

src/librustc_back/target/powerpc64_unknown_linux_gnu.rs

@@ -12,6 +12,7 @@
 
 pub fn target() -> Target {
     let mut base = super::linux_base::opts();
+    base.cpu = "ppc64".to_string();
     base.pre_link_args.push("-m64".to_string());
 
     Target {

src/librustc_back/target/powerpc64le_unknown_linux_gnu.rs

@@ -12,6 +12,7 @@
 
 pub fn target() -> Target {
     let mut base = super::linux_base::opts();
+    base.cpu = "ppc64le".to_string();
     base.pre_link_args.push("-m64".to_string());
 
     Target {