Add fiber asm for powerpc64, to avoid
"configure: error: setcontext required but not found"

Index: libphobos/configure
--- libphobos/configure.orig
+++ libphobos/configure
@@ -15196,7 +15196,7 @@ fi
     aarch64* | \
     arm* | \
     i[34567]86|x86_64 | \
-    powerpc)
+    powerpc*)
       druntime_fiber_asm_external=yes
       ;;
   esac