cmd/internal/obj/x86: add support for the endbr64 instruction on amd64

Add support for endbr64, which terminates an indirect branch in 64-bit
mode. This is effectively used to mark locations where an indirect branch
is permitted to land, when Indirect Branch Tracking (IBT) is enforced on
Intel CPUs.

Updates #66054

Change-Id: Ib898031711cfaaa6e05c197bfe727ded0bce6f52
Reviewed-on: https://go-review.googlesource.com/c/go/+/649215
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/cmd/asm/internal/asm/testdata/amd64enc_extra.s

@@ -1059,5 +1059,7 @@ TEXT asmtest(SB),DUPOK|NOSPLIT,$0
 	RDPID DX                                // f30fc7fa
 	RDPID R11                               // f3410fc7fb
 
+	ENDBR64					// f30f1efa
+
 	// End of tests.
 	RET

src/cmd/internal/obj/x86/aenum.go

@@ -212,6 +212,7 @@ const (
 	ADPPD
 	ADPPS
 	AEMMS
+	AENDBR64
 	AENTER
 	AEXTRACTPS
 	AF2XM1

src/cmd/internal/obj/x86/anames.go

@@ -210,6 +210,7 @@ var Anames = []string{
 	"DPPD",
 	"DPPS",
 	"EMMS",
+	"ENDBR64",
 	"ENTER",
 	"EXTRACTPS",
 	"F2XM1",

src/cmd/internal/obj/x86/asm6.go

@@ -1096,6 +1096,7 @@ var optab =
 	{ADPPD, yxshuf, Pq, opBytes{0x3a, 0x41, 0}},
 	{ADPPS, yxshuf, Pq, opBytes{0x3a, 0x40, 0}},
 	{AEMMS, ynone, Pm, opBytes{0x77}},
+	{AENDBR64, ynone, Pf3, opBytes{0x1e, 0xfa}},
 	{AEXTRACTPS, yextractps, Pq, opBytes{0x3a, 0x17, 0}},
 	{AENTER, nil, 0, opBytes{}}, // botch
 	{AFXRSTOR, ysvrs_mo, Pm, opBytes{0xae, 01, 0xae, 01}},