Allow __fastcall__ for C function. Contrary to the name, this is a size

Allow __fastcall__ for C function. Contrary to the name, this is a size
optimization for C functions. The last parameter is not pushed onto the
stack by the caller but in the function entry code, so there is no speed
but a size gain.

git-svn-id: svn://svn.cc65.org/cc65/trunk@645 b7a2c559-68d2-44c3-8de9-860c34a00d81