Retrieve segment registers on Windows amd64

Retrieve segment registers on Windows amd64

For amd64, CONTEXT_FULL does not contain CONTEXT_SEGMENTS, which seems
to be needed to retrieve all the segment registers.  Add it explicitly,
with a little de-cruftification.

The value of the segment registers isn't terribly useful on amd64, but
at least this makes the output of 'info registers' correct.

Before:

    (gdb)  i r cs ss ds es fs gs
    cs             0x33     51
    ss             0x2b     43
    ds             0x0      0
    es             0x0      0
    fs             0x0      0
    gs             0x0      0

After:

    (gdb) i r cs ss ds es fs gs
    cs             0x33     51
    ss             0x2b     43
    ds             0x2b     43
    es             0x2b     43
    fs             0x53     83
    gs             0x2b     43

gdb/ChangeLog

2015-02-27  Jon TURNEY  <jon.turney@dronecode.org.uk>

	* windows-nat.c (CONTEXT_DEBUGGER): Remove.
	(CONTEXT_DEBUGGER_DR): Add CONTEXT_SEGMENTS.  Incorporate flags
	from CONTEXT_DEBUGGER.