toolchain: introduce BR2_TOOLCHAIN_HAS_FULL_GETTEXT

This new boolean is true if the toolchain provides a built-in
full-featured implementation of gettext (glibc), and false if only a
stub implementation is provided (uclibc, musl).

This will be used in follow-up commits to decide whether libintl needs
to be built by gettext or not.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>