all: replace ReadRegister with AsmFull inline assembly

This makes AsmFull more powerful (by supporting return values) and
avoids a compiler builtin.

src/device/arm/arm.go

@@ -52,11 +52,10 @@ func Asm(asm string)
 //             "value":  1
 //             "result": &dest,
 //         })
-func AsmFull(asm string, regs map[string]interface{})
-
-// ReadRegister returns the contents of the specified register. The register
-// must be a processor register, reachable with the "mov" instruction.
-func ReadRegister(name string) uintptr
+//
+// You can use {} in the asm string (which expands to a register) to set the
+// return value.
+func AsmFull(asm string, regs map[string]interface{}) uintptr
 
 // Run the following system call (SVCall) with 0 arguments.
 func SVCall0(num uintptr) uintptr