[C++/mingw] gdbserver: gdb/host signal mixup

[C++/mingw] gdbserver: gdb/host signal mixup

Building in C++ caught a buglet here:

../../../src/gdb/gdbserver/win32-low.c: In function 'void win32_resume(thread_resume*, size_t)':
../../../src/gdb/gdbserver/win32-low.c:929:11: error: invalid conversion from 'int' to 'gdb_signal' [-fpermissive]
       sig = resume_info[0].sig;
           ^
../../../src/gdb/gdbserver/win32-low.c:934:11: error: invalid conversion from 'int' to 'gdb_signal' [-fpermissive]
       sig = 0;
           ^

Signals in the "struct thread_resume" structure are host signals, not
gdb signals.  The current code happens to work because the only
signals that the Windows port supports have the same number as the gdb
equivalent (see handle_exception for the win32 exception -> gdb signal
mapping).

gdb/gdbserver/ChangeLog:
2015-11-17  Pedro Alves  <palves@redhat.com>

	* win32-low.c (win32_resume): Use gdb_signal_from_host,
	GDB_SIGNAL_0 and gdb_signal_to_string.