Fix iOS build (fallout from 28b031c)

As Luke Iannini reported, the Clang iOS cross compiler apparently
doesn't support __thread for some bizarre reason, so unfortunately they
too must fall back to pthread_{get,set}specific.

Signed-off-by: Austin Seipp <austin@well-typed.com>

rts/sm/GCTDecl.h

@@ -53,8 +53,11 @@ extern StgWord8 the_gc_thread[];
 
 /* Now, llvm-gcc and some older Clang compilers do not support
    __thread. So we have to fallback to the extremely slow case,
-   unfortunately. Note: clang_CC_FLAVOR implies llvm_CC_FLAVOR */
-#if defined(llvm_CC_FLAVOR) && (CC_SUPPORTS_TLS == 0)
+   unfortunately. Note: clang_CC_FLAVOR implies llvm_CC_FLAVOR.
+
+   Also, the iOS Clang compiler doesn't support __thread either for
+   some bizarre reason, so there's not much we can do about that... */
+#if (defined(llvm_CC_FLAVOR) && (CC_SUPPORTS_TLS == 0)) || defined(ios_HOST_OS)
 #define gct ((gc_thread *)(pthread_getspecific(gctKey)))
 #define SET_GCT(to) (pthread_setspecific(gctKey, to))
 #define DECLARE_GCT ThreadLocalKey gctKey;