cmd/internal/obj: reduce per-architecture opcode space

s390x took up the last available chunk of int16 opcodes.
There are RISC-V and sparc64 ports in progress out of tree,
and there will likely be other architectures.
Reduce the opcode space to allow more architectures to
fit without increasing to int32.

This is the smallest power of two that accomodates all
existing architectures. All else being equal, smaller is
better--smaller numbers are easier to generate immediates
for and easier on the eyes when debugging.

Change-Id: I4d0824b28913892fbd0579d3f90bea34e44c8946
Reviewed-on: https://go-review.googlesource.com/24223
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/internal/obj/link.go

@@ -300,7 +300,7 @@ const (
 // Subspaces are aligned to a power of two so opcodes can be masked
 // with AMask and used as compact array indices.
 const (
-	ABase386 = (1 + iota) << 12
+	ABase386 = (1 + iota) << 10
 	ABaseARM
 	ABaseAMD64
 	ABasePPC64
@@ -308,7 +308,7 @@ const (
 	ABaseMIPS64
 	ABaseS390X
 
-	AllowedOpCodes = 1 << 12            // The number of opcodes available for any given architecture.
+	AllowedOpCodes = 1 << 10            // The number of opcodes available for any given architecture.
 	AMask          = AllowedOpCodes - 1 // AND with this to use the opcode as an array index.
 )