cmd/internal/obj: switch As back to int16

Upstream has fixed the opcode ranges to fit riscv in a 16-bit As, so we
can switch back.

Fixes golang#6

Change-Id: I1f18c477761be1801276e9f5f49c347e6ae33d91

Author: prattmic

src/cmd/internal/obj/link.go

@@ -266,7 +266,7 @@ type ProgInfo struct {
 // that are common to all architectures.
 // However, the majority of opcodes are arch-specific
 // and are declared in their respective architecture's subpackage.
-type As int32
+type As int16
 
 // These are the portable opcodes.
 const (