Rename CallConv::Native to CallConv::SystemV. (#291)

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.

cranelift/filetests/regalloc/schedule-moves.cton

@@ -1,7 +1,7 @@
 test regalloc
 isa intel haswell
 
-function %pr165() native {
+function %pr165() system_v {
 ebb0:
     v0 = iconst.i32 0x0102_0304
     v1 = iconst.i32 0x1102_0304
@@ -19,7 +19,7 @@ ebb0:
 
 ; Same as above, but use so many registers that spilling is required.
 ; Note: This is also a candidate for using xchg instructions.
-function %emergency_spill() native {
+function %emergency_spill() system_v {
 ebb0:
     v0 = iconst.i32 0x0102_0304
     v1 = iconst.i32 0x1102_0304