add support for -msave-args in gcc on amd64

i got sick of not having arguments in ddb stack traces on amd64,
which is because amd64 passes arguments in registers, and it's
impossible to figure out where they go without dwarf info, and when
you have dwarf info it is complicated.

solaris has a simple solution for this. they tweaked their compilers
to accept an -msave-args option which makes functions store their
arguments on the stack, while maintaining compatability with the
System V AMD64 ABI. tools (eg, ddb) can then look at the stack to
get access to function arguments in traces.

this ports their changes to gcc 3 to our gcc.

ok deraadt@

branches: 1.1.1;
Initial revision