[dev.regabi] cmd/asm: define g register on AMD64

Define g register as R14 on AMD64. It is not used now, but will
be in later CLs.

The name "R14" is still recognized.

Change-Id: I9a066b15bf1051113db8c6640605e350cea397b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/289195
Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/asm/internal/arch/arch.go

@@ -109,6 +109,10 @@ func archX86(linkArch *obj.LinkArch) *Arch {
 	register["SB"] = RSB
 	register["FP"] = RFP
 	register["PC"] = RPC
+	if linkArch == &x86.Linkamd64 {
+		// Alias g to R14
+		register["g"] = x86.REGG
+	}
 	// Register prefix not used on this architecture.
 
 	instructions := make(map[string]obj.As)

src/cmd/asm/internal/asm/operand_test.go

@@ -259,6 +259,7 @@ var amd64OperandTests = []operandTest{
 	{"R15", "R15"},
 	{"R8", "R8"},
 	{"R9", "R9"},
+	{"g", "R14"},
 	{"SI", "SI"},
 	{"SP", "SP"},
 	{"X0", "X0"},

src/cmd/internal/obj/x86/a.out.go

@@ -263,6 +263,7 @@ const (
 	FREGRET  = REG_X0
 	REGSP    = REG_SP
 	REGCTXT  = REG_DX
+	REGG     = REG_R14     // g register in ABIInternal
 	REGEXT   = REG_R15     // compiler allocates external registers R15 down
 	FREGMIN  = REG_X0 + 5  // first register variable
 	FREGEXT  = REG_X0 + 15 // first external register