x86_64: Initial varargs support.

 - Doesn't yet handle case where values are passed in mixed (general
   purpose & floating point) registers; otherwise largely
   functional. Code still needs some cleaning.

Fixes:
MultiSource/Applications/lua/lua
MultiSource/Applications/siod/siod
MultiSource/Applications/sqlite3/sqlite3
SingleSource/Regression/C/PR640
SingleSource/UnitTests/2003-07-09-SignedArgs
SingleSource/UnitTests/2007-03-02-VaCopy

gcc compat test suite results (Darwin x86-32 & -64):
--
# of expected passes		1262
# of unexpected failures	56
# of unresolved testcases	34
# of unsupported tests		2

Compare to: http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20090209/012050.html

llvm-svn: 64370