Fix building libcore for the Sony PSP

Building the MIPS MSA module for non-fp64 targets fails with an LLVM
error. This commit blacklists PSP targets from MSA support in order to
fix building libcore.

library/stdarch/crates/core_arch/src/mips/mod.rs

@@ -1,7 +1,12 @@
 //! MIPS
 
-mod msa;
-pub use self::msa::*;
+// Building this module (even if unused) for non-fp64 targets such as the Sony
+// PSP fails with an LLVM error. There doesn't seem to be a good way to detect
+// fp64 support as it is sometimes implied by the target cpu, so
+// `#[cfg(target_feature = "fp64")]` will unfortunately not work. This is a
+// fairly conservative workaround that only disables MSA intrinsics for the PSP.
+#[cfg(not(target_os = "psp"))] mod msa;
+#[cfg(not(target_os = "psp"))] pub use self::msa::*;
 
 #[cfg(test)]
 use stdarch_test::assert_instr;