arch/sparc: get rid of BR2_GCC_TARGET_TUNE

On SPARC, BR2_GCC_TARGET_TUNE was only used for one specific case, the
BR2_sparc_v8. There is actually no reason to not use
BR2_GCC_TARGET_CPU instead for this, as all values supported for
-mtune are also supported for -mcpu. Therefore, the only
BR2_GCC_TARGET_TUNE case is moved as a BR2_GCC_TARGET_CPU case.

[Peter: fixup merge conflict after 'sparc: Add leon3 cpu type and remove sparc{s,h}fleon{,v8}]
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>