Promote windows_core_xfer_shared_libraries and windows_core_pid_to_str

Promote windows_core_xfer_shared_libraries and windows_core_pid_to_str

Move windows_core_xfer_shared_libraries() and windows_core_pid_to_str()
to windows-tdep, and use in amd64-windows-tdep.c to handle Cygwin x86_64
core dumps.

v2:
Keep _initialize function at the bottom of the file.

gdb/ChangeLog:

2020-07-01  Jon Turney  <jon.turney@dronecode.org.uk>

	* windows-tdep.h: Add prototypes.
	* i386-windows-tdep.c(windows_core_xfer_shared_libraries): Move.
	(i386_windows_core_pid_to_str): Move and rename ...
	* windows-tdep.c (windows_core_xfer_shared_libraries): ... to here
	(windows_core_pid_to_str): ... and here.
	* amd64-windows-tdep.c (amd64_windows_init_abi_common): Register here.