[X86] Add opcode "udb" (permanent #UD in 64-bit mode)

[hpax]

The opcode D6 has been officially reserved as a single-byte permanent
undefined (#UD) opcode in 64-bit mode with the mnemonic UDB. This is
already the behavior of all known 64-bit implementations; this is thus
merely an official statement of forward compatibility and the
assignment of a mnemonic.

This will be documented in the next version of the Intel Software
Developer's Manual; in the meantime I DO speak officially for Intel on
this issue.

The x86 Advisory Council has ratified this decision, and so it is
expected to be honored across vendors, but I obviously cannot make any
official statement on any other vendor's behalf.

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: H. Peter Anvin (hpax)

The opcode D6 has been officially reserved as a single-byte permanent undefined (#UD) opcode in 64-bit mode with the mnemonic UDB. This is already the behavior of all known 64-bit implementations; this is thus merely an official statement of forward compatibility and the assignment of a mnemonic.

This will be documented in the next version of the Intel Software
Developer's Manual; in the meantime I DO speak officially for Intel on
this issue.

The x86 Advisory Council has ratified this decision, and so it is
expected to be honored across vendors, but I obviously cannot make any
official statement on any other vendor's behalf.