[libunwind] Support extended amd64 segment registers (#229176)

Raise the amd64 DWARF register limit to 59 and add RFLAGS, all six
segment selectors, FS.base, and GS.base to the register model. Capture
the extended state and restore it when resuming, using sigreturn() on
FreeBSD to restore selectors and bases together. Pack the state into the
existing context buffers without changing their public sizes or the
Windows XMM offsets.

Add tests for register access, signal-frame CFI recovery, register
bounds, and resume behavior.

Assisted-by: GPT
GitOrigin-RevId: fd4113508a64bb2dff7bbc530b41feaf33a9d08d