[gdb/tdep] Add XOP support in amd64_get_insn_details

Implement support for XOP instructions [1] in amd64_get_insn_details.

The encoding scheme is documented here [2].  Essentially it's a variant of the
VEX3 encoding scheme, with:
- 0x8f as the first byte instead of 0xc4, and
- an opcode map >= 8.

The changes are roughly the same as the XOP part of an earlier submission [3],
hence the tag.

The only real difference is that that patch proposed to implement xop_prefix_p
using:
...
  return pfx[0] == 0x8f && (pfx[1] & 0x38);
...
which tries to resolve the conflict between the XOP prefix (starts with 0x8f)
and the POP instruction (opcode 0x8f) by detecting that it's not a POP
instruction.

Instead, use the way AMD has resolved this conflict in the specification, by
checking for opcode map >= 8:
...
  gdb_byte m = pfx[1] & 0x1f;
  return pfx[0] == 0x8f && m >= 8;
...

Tested on x86_64-linux.

Co-Authored-By: Jan Beulich <jbeulich@suse.com>
Reviewed-By: Klaus Gerlicher<klaus.gerlicher.@intel.com>

[1] https://en.wikipedia.org/wiki/XOP_instruction_set
[2] https://www.amd.com/content/dam/amd/en/documents/archived-tech-docs/programmer-references/43479.pdf
[3] https://sourceware.org/pipermail/gdb-patches/2019-February/155347.html

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