x86: Add NOTRACK prefix support

For register indirect branches, NOTRACK prefix (0x3e), which is also
the DS segment register prefix, can be used to ignore the CET indirect
branch track.

gas/

	* config/tc-i386.c (REX_PREFIX): Changed to 7.
	(NOTRACK_PREFIX): New.
	(MAX_PREFIXES): Changed to 8.
	(_i386_insn): Add notrack_prefix.
	(PREFIX_GROUP): Add PREFIX_DS.
	(add_prefix): Return PREFIX_DS for DS_PREFIX_OPCODE.
	(md_assemble): Check if NOTRACK prefix is supported.
	(parse_insn): Set notrack_prefix and issue an error for
	other prefixes after NOTRACK prefix.
	* testsuite/gas/i386/i386.exp: Run tests for NOTRACK prefix.
	* testsuite/gas/i386/notrack-intel.d: New file.
	* testsuite/gas/i386/notrack.d: Likewise.
	* testsuite/gas/i386/notrack.s: Likewise.
	* testsuite/gas/i386/notrackbad.l: Likewise.
	* testsuite/gas/i386/notrackbad.s: Likewise.
	* testsuite/gas/i386/x86-64-notrack-intel.d: Likewise.
	* testsuite/gas/i386/x86-64-notrack.d: Likewise.
	* testsuite/gas/i386/x86-64-notrack.s: Likewise.
	* testsuite/gas/i386/x86-64-notrackbad.l: Likewise.
	* testsuite/gas/i386/x86-64-notrackbad.s: Likewise.

include/

	* include/opcode/i386.h (NOTRACK_PREFIX_OPCODE): New.

opcodes/

	* i386-dis.c (NOTRACK_Fixup): New.
	(NOTRACK): Likewise.
	(NOTRACK_PREFIX): Likewise.
	(last_active_prefix): Likewise.
	(reg_table): Use NOTRACK on indirect call and jmp.
	(ckprefix): Set last_active_prefix.
	(prefix_name): Return "notrack" for NOTRACK_PREFIX.
	* i386-gen.c (opcode_modifiers): Add NoTrackPrefixOk.
	* i386-opc.h (NoTrackPrefixOk): New.
	(i386_opcode_modifier): Add notrackprefixok.
	* i386-opc.tbl: Add NoTrackPrefixOk to indirect call and jmp.
	Add notrack.
	* i386-tbl.h: Regenerated.

include/opcode/i386.h

@@ -77,6 +77,7 @@
 #define XACQUIRE_PREFIX_OPCODE 0xf2
 #define XRELEASE_PREFIX_OPCODE 0xf3
 #define BND_PREFIX_OPCODE 0xf2
+#define NOTRACK_PREFIX_OPCODE 0x3e
 
 #define TWO_BYTE_OPCODE_ESCAPE 0x0f
 #define NOP_OPCODE (char) 0x90