apple: build.sh: add bitcode flag to LDFLAGS too

The bitcode flag was already added to CFLAGS, but was only used when
creating a static build of libvlc, which doesn't involve linkage. When
creating shared plugins, the dylibs must be linked with the
-fembed-bitcode flag to gather the bitcode from the different objects.

In particular, bitcode is mandatory on tvOS, so this is mandatory when
building dynamic plugins and/or libvlc for tvOS.