Fix mingw build of gdb

Fix mingw build of gdb

Christian pointed out on irc that the threading series broke the build
on mingw.  This patch fixes the problem, by moving the initialization
of gdb_demangle_attempt_core_dump into the appropriate #if.

gdb/ChangeLog
2019-11-26  Tom Tromey  <tromey@adacore.com>

	* cp-support.c (_initialize_cp_support): Conditionally initialize
	gdb_demangle_attempt_core_dump.

Change-Id: I9ace0bea75a51f317ea933b607f6b5a94d651eea