rustc_target: spec: Ensure that a valid llvm_abiname value is set for PowerPC64(LE)

PowerPC64 ELF targets (effectively anything that isn't AIX) use either
the ELFv1 or ELFv2 ABI. The ELFv1 ABI is only specified for big endian
targets, while ELFv2 can be used by both little- and big-endian targets.
Make sure that, if an LLVM ABI is set, it is set to one of the two. AIX
does not set an LLVM ABI name, so it does not need to be accounted for
in any way other than allowing an unset value.

Author: Gelbpunkt

compiler/rustc_target/src/spec/mod.rs

@@ -3182,6 +3182,22 @@ impl Target {
                     "ARM targets must set `llvm-floatabi` to `hard` or `soft`",
                 )
             }
+            // PowerPC64 targets that are not AIX must set their ABI to either ELFv1 or ELFv2
+            Arch::PowerPC64 if self.os != Os::Aix => {
+                if self.endian == Endian::Big {
+                    check_matches!(
+                        &*self.llvm_abiname,
+                        "elfv1" | "elfv2",
+                        "invalid PowerPC64 ABI name: {}",
+                        self.llvm_abiname,
+                    );
+                } else {
+                    check!(
+                        self.llvm_abiname == "elfv2",
+                        "little-endian PowerPC64 targets only support the `elfv2` ABI",
+                    );
+                }
+            }
             _ => {}
         }