Add -mdynamic-no-pic to compile flags for gcc. On Mac OS X and Darwin,

the compiler generates position-independent-code by default.  This allows
an object file to be used for a dylib or executable without any thought
on the developer's part, but generates less-efficient code.  Setting this
flag on gcc speeds compiles by 3-5%.
        * config/rs6000/x-darwin: Add -mdynamic-no-pic to gcc build flags.

From-SVN: r86141