arm: implement SVCall

The SVCall function call is lowered to an inline "svc" instruction. The
first parameter *must* be a constant number.

src/device/arm/arm.go

@@ -54,6 +54,18 @@ func AsmFull(asm string, regs map[string]interface{})
 // must be a processor register, reachable with the "mov" instruction.
 func ReadRegister(name string) uintptr
 
+// Run the following system call (SVCall) with 1 argument.
+func SVCall1(num uintptr, a1 interface{}) uintptr
+
+// Run the following system call (SVCall) with 2 arguments.
+func SVCall2(num uintptr, a1, a2 interface{}) uintptr
+
+// Run the following system call (SVCall) with 3 arguments.
+func SVCall3(num uintptr, a1, a2, a3 interface{}) uintptr
+
+// Run the following system call (SVCall) with 4 arguments.
+func SVCall4(num uintptr, a1, a2, a3, a4 interface{}) uintptr
+
 //go:volatile
 type RegValue uint32