Avoid adding all kinds of things to the toolchain's flags that within
Gentoo (Prefix) we really shouldn't, such as sysroot, deployment target,
arch, etc.

--- a/tools/darwin.jam
+++ b/tools/darwin.jam
@@ -240,6 +240,9 @@
         }
     }
 
+    # leave compiler flags etc. up to the toolchain
+    return $(version-feature) ;
+
     if $(version-feature)
     {
         if $(.debug-configuration)
@@ -448,7 +451,8 @@
             support-ppc64 = ;
         }
     }
-    switch $(arch)
+    # Gentoo Prefix toolchain doesn't do multi-arch, so don't try either
+    switch $(donotaddarchpleaseXXXarch)
     {
         case combined :
         {