package/efivar: needs host gcc >= 4.8

The efivar code compiled for the host machine uses
__builtin_bswap16(), which is only available starting from gcc 4.8:

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52624

So let's add a dependency on host gcc >= 4.8 to efivar and its unique
reverse dependency, efibootmgr.

Fixes:

  http://autobuild.buildroot.net/results/48ba906bb6f4dc0c8af43ec11be64f7168dd62fd/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>