Add Windows x64 SEH unwinder (take 2)

[Joel Brobecker <brobecker at adacore dot com>]

Hello,

This patch series adds an unwinder using the native Windows x64 SEH
data. This dramatically improves the behavior as soon as system
calls are involved. For instance, after switching to a different
thread, we now get...

    (gdb) bt
    #0  0x0000000077a46eba in ntdll!ZwWaitForSingleObject ()
       from C:\Windows\system32\ntdll.dll
    #1  0x00000000778fb380 in WaitForSingleObjectEx ()
       from C:\Windows\system32\kernel32.dll
    #2  0x000000000040a7e9 in system__task_primitives__operations__cond_wait ()
    #3  0x0000000000407c3b in system__tasking__rendezvous__accept_trivial ()
    #4  0x0000000000401f3c in task_switch.callee (<_task>=0x394410)
        at task_switch.adb:29
    #5  0x0000000000406637 in system__tasking__stages__task_wrapper ()
    [...]

... instead of:

    #0  0x0000000077a46eba in ntdll!ZwWaitForSingleObject ()
       from C:\Windows\system32\ntdll.dll
    #1  0x00000000778fb380 in WaitForSingleObjectEx ()
       from C:\Windows\system32\kernel32.dll
    #2  0x000007fffffdc000 in ?? ()
    #3  0x0000000077a266e4 in ntdll!LdrInitializeThunk ()
       from C:\Windows\system32\ntdll.dll
    #4  0x0000000000000000 in ?? ()