cmd/internal/obj/arm64: add support of NOOP instruction

This patch uses symbol NOOP to support arm64 instruction NOP. In
arm64, NOP stands for that No Operation does nothing, other than
advance the value of the program counter by 4. This instruction
can be used for instruction alignment purposes. This patch uses
NOOP to support arm64 instruction NOP, because we have a generic
"NOP" instruction, which is a zero-width pseudo-instruction.

In arm64, instruction NOP is an alias of HINT #0. This patch adds
test cases for instruction HINT #0.

Change-Id: I54e6854c46516eb652b412ef9e0f73ab7f171f8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/200578
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/asm/internal/asm/testdata/arm64.s

@@ -1038,6 +1038,8 @@ again:
 	FSTPS	(F3, F4), 1024(RSP) // fb0310916313002d
 	FSTPS	(F3, F4), x(SB)
 	FSTPS	(F3, F4), x+8(SB)
+	NOOP                        // 1f2003d5
+	HINT $0                     // 1f2003d5
 
 // System Register
 	MSR	$1, SPSel                          // bf4100d5

src/cmd/asm/internal/asm/testdata/arm64enc.s

@@ -152,6 +152,7 @@ TEXT asmtest(SB),DUPOK|NOSPLIT,$-8
    EXTR $35, R22, R12, R8                     // 888dd693
    SEVL                                       // bf2003d5
    HINT $6                                    // df2003d5
+   HINT $0                                    // 1f2003d5
    HLT $65509                                 // a0fc5fd4
    HVC $61428                                 // 82fe1dd4
    ISB $1                                     // df3103d5
@@ -281,7 +282,7 @@ TEXT asmtest(SB),DUPOK|NOSPLIT,$-8
    NGC R2, R7                                 // e70302da
    NGCSW R10, R5                              // e5030a7a
    NGCS R24, R16                              // f00318fa
-   //TODO NOP                                 // 1f2003d5
+   NOOP                                        // 1f2003d5
    ORNW R4@>11, R16, R3                       // 032ee42a
    ORN R22@>19, R3, R3                        // 634cf6aa
    ORRW $4294443071, R15, R24                 // f8490d32

src/cmd/internal/obj/arm64/a.out.go

@@ -701,6 +701,7 @@ const (
 	ANGCS
 	ANGCSW
 	ANGCW
+	ANOOP
 	AORN
 	AORNW
 	AORR

src/cmd/internal/obj/arm64/asm7.go

@@ -2486,6 +2486,7 @@ func buildop(ctxt *obj.Link) {
 			oprangeset(AYIELD, t)
 			oprangeset(ASEV, t)
 			oprangeset(ASEVL, t)
+			oprangeset(ANOOP, t)
 			oprangeset(ADRPS, t)
 
 		case ACBZ:
@@ -6036,8 +6037,8 @@ func (c *ctxt7) op0(p *obj.Prog, a obj.As) uint32 {
 	case AERET:
 		return 0x6B<<25 | 4<<21 | 0x1F<<16 | 0<<10 | 0x1F<<5
 
-	// case ANOP:
-	// 	return SYSHINT(0)
+	case ANOOP:
+		return SYSHINT(0)
 
 	case AYIELD:
 		return SYSHINT(1)