Windows gdb+gdbserver: Share $_siginfo reading code

Both GDB and GDBserver have similar code to read the $_siginfo data.
This patch moves the bulk of it to gdb/nat/windows-nat.c so it can be
shared.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: I58f9074caf6362274453c78ed1fc9e31249f6854
4 files changed