[osx] build fix for make (tests etc.) on darwin with llvm.

The asm we're using for truncate_int() (largely unused) is no-worky.

Note that this was like this before https://github.com/xbmc/xbmc/pull/3760 was merged.

Ideally we'd have an SSE implementation here, or just kill the ASM which is likely slower than just truncating directly.

@bavison any comment on the speed of the truncate_int() implementation?

(It's only used in GUIFontTTF.cpp, and can probably be removed altogether which is another option).