arch/x86: get rid of BR2_GCC_TARGET_TUNE

According to the gcc documentation, -march implies -mtune on x86, so
defining both BR2_GCC_TARGET_ARCH and BR2_GCC_TARGET_TUNE is
redundant. Therefore, this commit removes the definition of
BR2_GCC_TARGET_TUNE on x86.

However, while doing so, it adds one new case for BR2_GCC_TARGET_ARCH:
using -march=generic when BR2_x86_generic is selected.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>