Hello,

This patch rewrites ppc_svr4_push_dummy_call so that it implements the 
two-pass push-arguments algorithm using a "for" loop.

The simplification identified and fixed the bugs:

- Altivec registers on the stack weren't being 16 byte aligned
- Floats weren't being converted to doubles before being stored in 
register/memory ("gdb.base/callfuncs.exp: p t_float_values2" now passes).

Ok?

Andrew

PS: I intend moving align_{up,down} to utils.c but that is separate.