-Wwrite-strings: gdbserver/win32-low.c and TARGET_WAITKIND_EXECD

-Wwrite-strings: gdbserver/win32-low.c and TARGET_WAITKIND_EXECD

 src/gdb/gdbserver/win32-low.c:1499:39: error: ISO C++ forbids converting a string constant to 'char*' [-Werror=write-strings]
	ourstatus->value.execd_pathname = "Main executable";
					^

This reporting via TARGET_WAITKIND_EXECD it's totally unnecessary.
get_child_debug_event returns a TARGET_WAITKIND_SPURIOUS by default,
which works just as well here, and is what the equivalent code in
gdb/windows-nat.c does too.

gdb/gdbserver/ChangeLog:
2017-04-05  Pedro Alves  <palves@redhat.com>

	* win32-low.c (get_child_debug_event)
	<CREATE_PROCESS_DEBUG_EVENT>: Don't report TARGET_WAITKIND_EXECD.
	Report TARGET_WAITKIND_SPURIOUS instead.