Introduce likely() and unlikely() macros, use to give gcc hints about which branch...

Introduce likely() and unlikely() macros, use to give gcc hints about which branch is likely to be taken in a conditional, use them in the midi player for a small speedup, use instead of similar EXPECT macros in tremor and use in mpegplayer

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@19162 a1c6a512-1295-4272-9138-f99709370657