Extend UTF-8 support to the 32-bit mingw host.

Prevent any name mangling in HOST_EXTRA_OBJS_SYMBOL
such that the linker always finds it by that name.

Also add the .manifest file as an explicit
dependency in the make rule such that the
object gets re-built if it changes.

gcc/ChangeLog:

	* config.host: Pull in i386/x-mingw32-utf8 Makefile
	fragment and reference utf8rc-mingw32.o explicitly
	for mingw hosts.
	* config/i386/sym-mingw32.cc: prevent name mangling of
	stub symbol.
	* config/i386/x-mingw32-utf8: Make utf8rc-mingw32.o
	depend on manifest file explicitly.

Signed-off-by: Jonathan Yong <10walls@gmail.com>