added notes on the fact that the current implementation uses
sbyte* for va_list.  Updated all occurances of valist to va_list (it was
inconsistant and find/replace is so easy ;-) ).  Added <...> around all
occurances of va_list in the intrinsic functions to match the vaarg and
vanext instructions and to further show that va_list is a variable type.

llvm-svn: 16530