Fix PowerPC build

Summary:
Replace incorrect `ppc_HOST_ARCH` with `powerpc_HOST_ARCH`.

Test Plan: Build on PowerPC

Reviewers: hvr, austin, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1651

libraries/ghci/GHCi/InfoTable.hsc

@@ -63,7 +63,7 @@ platform :: Arch
 platform =
 #if defined(sparc_HOST_ARCH)
        ArchSparc
-#elif defined(ppc_HOST_ARCH)
+#elif defined(powerpc_HOST_ARCH)
        ArchPPC
 #elif defined(x86_HOST_ARCH)
        ArchX86