The main use for non-PIC code at present is JIT code, and JIT code can
live anywhere in memory and reference other symbols defined anywhere in
memory, so it needs to use the "large" code model.
func_addr and globalsym_addr instructions were already using `movabs`
to support arbitrary 64-bit addresses, so this just makes calls be
legalized to support arbitrary 64-bit addresses also.
To keep cross-compiling straightforward, Cretonne shouldn't have any
behavior that depends on the host. This renames the "Native" calling
convention to "SystemV", which has a defined meaning for each target,
so that it's clear that the calling convention doesn't change
depending on what host Cretonne is running on.
