binutils: disable gold on mingw

binutils: disable gold on mingw

oe-core 759eed (binutils: Enable threading when gold is enabled and is not
default linker) causes linking in mingw SDKs to fail:

.../work/i686-nativesdk-mingw32-pokysdk-mingw32/binutils-cross-canadian-x86-64/2.28-r0
/recipe-sysroot-native/usr/bin/i686-pokysdk-mingw32/../../libexec/i686-pokysdk-mingw32/gcc/i686-pokysdk-mingw32/6.3.0/ld:
cannot find -lpthread

Work around this by disabling gold entirely in mingw SDKs.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>