[gdb/tdep] Support REX2 and EVEX prefix

The following amd64 insn:
...
   0:	67 d5 44 8d 3d 00 00 00 00 lea 0x0(%eip),%r31d
...
uses the REX2 prefix [1], which is currently not supported in
amd64_get_insn_details.

Add the missing support in amd64_get_insn_details, as well as a corresponding
unit test.

Likewise for an amd64 insn using an EVEX prefix [2]:
...
   0:	62 f1 7c 48 28 05 00 fc ff ff vmovaps -0x400(%rip),%zmm0
...

Tested on x86_64-linux.

PR tdep/32725
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32725

[1] https://en.wikipedia.org/wiki/VEX_prefix
[2] https://en.wikipedia.org/wiki/EVEX_prefix