Add read_pc / write_pc support to win32-low

Add read_pc / write_pc support to win32-low

This changes win32-low.c to implement the read_pc and write_pc
methods.  A subsequent patch will need these.

Note that I have no way to test, or even compile, the win32-arm-low.c
change.

gdbserver/ChangeLog
2020-04-08  Tom Tromey  <tromey@adacore.com>

	* win32-low.h (win32_process_target::read_pc)
	(win32_process_target::write_pc): Declare.
	* win32-low.c (win32_process_target::read_pc)
	(win32_process_target::write_pc): New methods.
	* win32-i386-low.c (i386_win32_get_pc, i386_win32_set_pc): New
	functions.
	(the_low_target): Update.
	* win32-arm-low.c (arm_win32_get_pc, arm_win32_set_pc): New
	functions.
	(the_low_target): Update.