cmd/internal/obj: add support for s390x

Adds a new R_PCRELDBL relocation for 2-byte aligned relative
relocations on s390x. Should be removed once #14218 is
implemented.

Change-Id: I79dd2d8e746ba8cbc26c570faccfdd691e8161e8
Reviewed-on: https://go-review.googlesource.com/20941Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/link.go

@@ -299,6 +299,7 @@ const (
 	ABasePPC64
 	ABaseARM64
 	ABaseMIPS64
+	ABaseS390X
 
 	AMask = 1<<12 - 1 // AND with this to use the opcode as an array index.
 )
@@ -564,6 +565,10 @@ const (
 	// R_ADDRPOWER_DS but inserts the offset from the TOC to the address of the the
 	// relocated symbol rather than the symbol's address.
 	R_ADDRPOWER_TOCREL_DS
+
+	// R_PCRELDBL relocates s390x 2-byte aligned PC-relative addresses.
+	// TODO(mundaym): remove once variants can be serialized - see issue 14218.
+	R_PCRELDBL
 )
 
 type Auto struct {

src/cmd/internal/obj/util.go

@@ -529,6 +529,7 @@ const (
 	RBasePPC64  = 4 * 1024  // range [4k, 8k)
 	RBaseARM64  = 8 * 1024  // range [8k, 13k)
 	RBaseMIPS64 = 13 * 1024 // range [13k, 14k)
+	RBaseS390X  = 14 * 1024 // range [14k, 15k)
 )
 
 // RegisterRegister binds a pretty-printer (Rconv) for register