PPC NCG: Emit more portable `fcmpu 0, ...` instead of `fcmpu cr0, ...`
PPC NCG: Emit more portable `fcmpu 0, ...` instead of `fcmpu cr0, ...`

Use `fcmpu 0, ...` rather than `fcmpu cr0, ...` for better
portability since some non-GNU assembler (such as IBM's `as`) tend to not
support the symbolic register name `cr0`. This matches the syntax that
GCC emits for PPC targets.

(cherry picked from commit 6f0e41da96b79cde49ad8761738c281cd190ac65)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc