gcc: fix Objective-C support

This commit fixes the Objective-C support in the gcc package by making
sure that the runtime Objective-C library is properly installed to the
target.

It also takes this opportunity to slightly reword the Config.in option
help text to have a more meaningful explanation.

Finally, it fixes the gcc-final.mk code to use the correct
BR2_INSTALL_OBJC option rather than the non-existing
BR2_GCC_CROSS_OBJC option, to decide whether Objective-C support
should be enabled or not in the cross-compiler.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>