Fix native MSYS2 build failure [PR108865, PR109188]

Fix native MSYS2 build failure [PR108865, PR109188]

Don't cause an error if the symbol is not found.

When building natively as an MSYS2 package, some
executables are borrowing the $(COMPILERS) flags
and --require-defined=HOST_EXTRA_OBJS_SYMBOL is
causing them to fail.

These executables don't need that symbol, so make
it optional such that they don't error out.

The compilers will still look for it (and find it)
with the desired effect of getting UTF-8 support.

	PR/PR108865

gcc/ChangeLog:

	* config/i386/x-mingw32-utf8: Make HOST_EXTRA_OBJS_SYMBOL
	optional.

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