Specific calling convention for variadic arguments

[toshipiazza]

Is your feature request related to a problem? Please describe.

The Qualcomm Hexagon Application Binary Interface specifies a unique calling convention for functions that take variadic arguments:

For [variadic] functions, we pass the named (and typed) parameters in the same manner as for fixed argument list functions. The remainder of the arguments are passed on the stack.

This calling convention differs from most calling conventions I've seen in other architectures, for example:

int snprintf(char *str, char *format, ...);

The first argument str is passed in r0. The second argument format is passed in r1. All subsequent (variadic) arguments are passed on the stack. Whereas the calling convention for ARM would be to enregister all variadic arguments as well.

Describe the solution you'd like

I'd like the cspec to have an extra field to denote that variadic arguments are put on the stack

Alternatively, I'd like to be able to specify a function signature override that can specify custom storage in an Analyzer or from the GUI

Alternatively, I'd like to be able to create a completely custom function prototype/calling convention in an Analyzer so I could create a new calling convention for each variadic function

Additional context

XREF #1595
XREF toshipiazza/ghidra-plugin-hexagon#2