tcg/ppc: Handle _CALL_DARWIN being undefined on Darwin

powerpc-apple-darwin9-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5577)
does not define _CALL_DARWIN, leading to unexpected behavior w.r.t.
register clobbering and stack frame layout.

Since _CALL_DARWIN is a reserved identifier, define a custom
TCG_TARGET_CALL_DARWIN based on either _CALL_DARWIN or __APPLE__.

Signed-off-by: Andreas F?rber <andreas.faerber@web.de>
Signed-off-by: malc <av1474@comtv.ru>
