Fix debugging of WOW64 processes

The new code regarding pending stops only checks for EXCEPTION_BREAKPOINT,
but for WOW64 processes STATUS_WX86_BREAKPOINT is necessary as well.

Also, ignore_first_breakpoint is used now in nat/windows-nat.c as well,
but was not available there.

gdb/ChangeLog:

2020-04-10  Hannes Domani  <ssbssa@yahoo.de>

	* nat/windows-nat.c (STATUS_WX86_BREAKPOINT, STATUS_WX86_SINGLE_STEP):
	Move to...
	* nat/windows-nat.h (STATUS_WX86_BREAKPOINT, STATUS_WX86_SINGLE_STEP):
	... here.
	* windows-nat.c (windows_nat_target::get_windows_debug_event):
	Check for STATUS_WX86_BREAKPOINT.
	(windows_nat_target::wait): Same.