gdb: stop using host-dependent signal numbers in windows-tdep.c

gdb: stop using host-dependent signal numbers in windows-tdep.c

The signal enumeration in windows-tdep.c is defined differently whether
it is compiled on Cygwin or not.  This is problematic, since the code in
tdep files is not supposed to be influenced by the host platform (the
platform GDB itself runs on).

This makes a difference in windows_gdb_signal_to_target.  An obvious
example of clash is SIGABRT.  Let's pretend we are cross-debugging a
Cygwin process from a MinGW (non-Cygwin Windows) GDB.  If GDB needs to
translate the gdb signal number GDB_SIGNAL_ABRT into a target
equivalent, it would obtain the MinGW number (22), despite the target
being a Cygwin process.  Conversely, if debugging a MinGW process from a
Cygwin-hosted GDB, GDB_SIGNAL_ABRT would be converted to a Cygwin signal
number (6) despite the target being a MinGW process.  This is wrong,
since we want the result to depend on the target's platform, not GDB's
platform.

This known flaw was accepted because at the time we had a single OS ABI
(called Cygwin) for all Windows binaries (Cygwin ones and non-Cygwin
ones).  This limitation is now lifted, as we now have separate Windows
and Cygwin OS ABIs.  This means we are able to detect at runtime whether
the binary we are debugging is a Cygwin one or non-Cygwin one.

This patch splits the signal enum in two, one for the MinGW flavors and
one for Cygwin, removing all the ifdefs that made it depend on the host
platform.  It then makes two separate gdb_signal_to_target gdbarch
methods, that are used according to the OS ABI selected at runtime.

There is a bit of re-shuffling needed in how the gdbarch'es are
initialized, but nothing major.

gdb/ChangeLog:

	* windows-tdep.h (windows_init_abi): Add comment.
	(cygwin_init_abi): New declaration.
	* windows-tdep.c: Split signal enumeration in two, one for
	Windows and one for Cygwin.
	(windows_gdb_signal_to_target): Only deal with signal of the
	Windows OS ABI.
	(cygwin_gdb_signal_to_target): New function.
	(windows_init_abi): Rename to windows_init_abi_common, don't set
	gdb_signal_to_target gdbarch method.  Add new new function with
	this name.
	(cygwin_init_abi): New function.
	* amd64-windows-tdep.c (amd64_windows_init_abi_common): Add
	comment.  Don't call windows_init_abi.
	(amd64_windows_init_abi): Add comment, call windows_init_abi.
	(amd64_cygwin_init_abi): Add comment, call cygwin_init_abi.
	* i386-windows-tdep.c (i386_windows_init_abi): Rename to
	i386_windows_init_abi_common, don't call windows_init_abi.  Add
	a new function of this name.
	(i386_cygwin_init_abi): New function.
	(_initialize_i386_windows_tdep): Bind i386_cygwin_init_abi to
	OS ABI Cygwin.