TCG: Mac OS X support for ppc64 target

TCG: Mac OS X support for ppc64 target

Darwin/ppc64 does not use function descriptors,
adapt prologue and tcg_out_call accordingly.
GPR2 is available for general use, so let's use it.

http://developer.apple.com/mac/library/documentation/DeveloperTools/Conceptual/LowLevelABI/110-64-bit_PowerPC_Function_Calling_Conventions/64bitPowerPC.html

v2:
- Don't mark reserved GPR13 as callee-save.
- Move tcg_out_b up.
- Fix unused variable warning in prologue.

Signed-off-by: Andreas Faerber <andreas.faerber@web.de>
Cc: malc <av1474@comtv.ru>
Signed-off-by: malc <av1474@comtv.ru>