Fix GDB MI register parsing for XMM/YMM and unavailable values

[xusheng6]

Summary

• Handle <unavailable> register values gracefully (return 0 instead of logging an error)
• Parse XMM composite struct values by extracting the uint128 = 0x... field
• Parse YMM composite struct values by combining v2_int128 = {0xHIGH, 0xLOW} fields into a 256-bit value

Test plan

• Attach to a gdbserver target and verify no "Failed to parse GDB value" errors for vector registers
• Verify XMM register values are correctly displayed
• Verify YMM register values are correctly displayed
• Test with a target that has unavailable registers

🤖 Generated with Claude Code

[xusheng6]

Fixes: #1021